diff --git a/cggs.py b/cggs.py
new file mode 100644
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--- /dev/null
+++ b/cggs.py
@@ -0,0 +1,345 @@
+#!/usr/bin/env python3
+
+# SPDX-FileCopyrightText: © 2020 Open Networking Foundation <support@opennetworking.org>
+# SPDX-License-Identifier: Apache-2.0
+
+from __future__ import absolute_import
+
+import argparse
+import json
+import logging
+import urllib.request
+import urllib.parse
+import base64
+
+# create shared logger
+logging.basicConfig()
+logger = logging.getLogger("cg")
+
+# global dict of jsonpath expressions -> compiled jsonpath parsers, as
+# reparsing expressions in each loop results in 100x longer execution time
+gjpaths = {}
+
+# gerrit headers is cached
+gerrit_headers = []
+
+# gerrit_groups is a cache of groups, to prevent multiple lookups
+gerrit_groups = {}
+
+
+def parse_crowdgroups_args():
+    """
+    parse CLI arguments
+    """
+
+    parser = argparse.ArgumentParser(description="Crowd-Gerrit group sync")
+
+    # Positional args
+    parser.add_argument(
+        "settings", type=argparse.FileType("r"), help="JSON settings file",
+    )
+
+    parser.add_argument(
+        "--debug", action="store_true", help="Print additional debugging information"
+    )
+
+    return parser.parse_args()
+
+
+def basic_auth_header(username, password):
+    """
+    returns a tuple containing a http basic auth header
+    """
+    creds_str = "%s:%s" % (username, password)
+    creds_b64 = base64.standard_b64encode(creds_str.encode("utf-8"))
+
+    return ("Authorization", "Basic %s" % creds_b64.decode("utf-8"))
+
+
+def json_api_get(url, headers, data=None, trim_prefix=False, allow_failure=False):
+    """
+    Call JSON API endpoint, return data as a dict
+    """
+
+    # if data included, encode it as JSON
+    if data:
+        data_enc = str(json.dumps(data)).encode("utf-8")
+
+        request = urllib.request.Request(url, data=data_enc, method="POST")
+        request.add_header("Content-Type", "application/json; charset=UTF-8")
+    else:
+        request = urllib.request.Request(url)
+
+    # add headers tuples
+    for header in headers:
+        request.add_header(*header)
+
+    try:
+        response = urllib.request.urlopen(request)
+    except urllib.error.HTTPError:
+        # asking for data that doesn't exist results in a 404, just return nothing
+        if allow_failure:
+            return None
+        logger.exception("Server encountered an HTTPError at URL: '%s'", url)
+    except urllib.error.URLError:
+        logger.exception("An URLError occurred at URL: '%s'", url)
+    else:
+        # docs: https://docs.python.org/3/library/json.html
+        jsondata = response.read()
+        logger.debug("API response: %s", jsondata)
+
+    # optionally remove the gerrit "magic prefix" - docs:
+    # https://gerrit-review.googlesource.com/Documentation/rest-api.html#output
+    if trim_prefix:
+        jsondata = jsondata.lstrip(b")]}'\n")
+
+    try:
+        data = json.loads(jsondata)
+    except json.decoder.JSONDecodeError:
+        # allow return of no data
+        if allow_failure:
+            return None
+        logger.exception("Unable to decode JSON")
+    else:
+        logger.debug("JSON decoded: %s", data)
+
+    return data
+
+
+def crowd_users_list(crowd_settings, groupname):
+    """
+    Returns dict of email addresses mapped to usernames from Crowd, given a
+    group name
+    """
+
+    url = "%s/rest/usermanagement/1/group/user/direct?groupname=%s&expand=user" % (
+        crowd_settings["url"],
+        groupname,
+    )
+
+    crowd_headers = [
+        basic_auth_header(crowd_settings["username"], crowd_settings["password"]),
+        ("Accept", "application/json"),
+    ]
+
+    cout = json_api_get(url, crowd_headers)
+
+    userlist = {}
+
+    for user in cout["users"]:
+        userlist[user["email"]] = user["name"]
+
+    return userlist
+
+
+def gerrit_group_id_by_name(gerrit_settings, groupname):
+    """
+    Returns a gerrit group ID given the name of a group
+    Note - this is the "id" not the "group_id" in the dict.
+    """
+
+    # read from cache if found
+    if groupname not in gerrit_groups:
+
+        gurl = "%s/groups/?m=%s" % (gerrit_settings["url"], groupname)
+        gout = json_api_get(gurl, gerrit_headers, None, True)
+
+        gerrit_groups[groupname] = gout[groupname]["id"]
+
+        logger.debug(
+            "Group: '%s' has group-id: '%s'", groupname, gerrit_groups[groupname]
+        )
+
+    return gerrit_groups[groupname]
+
+
+def gerrit_users_list(gerrit_settings, groupname):
+    """
+    Returns dict of lists of usernames and emails from Gerrit, given a group name
+    """
+
+    gid = gerrit_group_id_by_name(gerrit_settings, groupname)
+
+    gurl = "%s/groups/%s/members" % (gerrit_settings["url"], gid)
+    gout = json_api_get(gurl, gerrit_headers, None, True)
+
+    userlist = {}
+
+    for user in gout:
+        userlist[user["email"]] = {
+            "_account_id": user.get("_account_id"),
+            "username": user.get("username", None),
+        }
+
+    return userlist
+
+
+def gerrit_check_crowd_id(gerrit_settings, aid, crowdusername):
+    """
+    checks if a gerrit account has an external CrowdID identity string
+    """
+
+    gurl = "%s/accounts/%s/external_ids" % (gerrit_settings["url"], aid)
+    gout = json_api_get(gurl, gerrit_headers, None, True)
+
+    crowd_id_string = (
+        "https://crowd.opennetworking.org/openidserver/users/%s" % crowdusername
+    )
+
+    for identity in gout:
+        if identity["identity"] == crowd_id_string:
+            return aid
+
+    return None
+
+
+def gerrit_find_account_id(gerrit_settings, email, crowdusername):
+    """
+    Returns a gerrit account ID given the email address of a user and the
+    corresponding crowd username, or None if no account exists.
+    """
+
+    gurl = "%s/accounts/?q=email:%s" % (gerrit_settings["url"], email)
+    gout = json_api_get(gurl, gerrit_headers, None, True, True)
+
+    # check for users with multiple email addresses
+    if len(gout) > 1:
+        logger.warning("Email: '%s' register for more than one Gerrit account", email)
+        return None
+
+    # may not have an account, return None if no account
+    if len(gout) == 0:
+        logger.warning("Email: '%s' doesn't have a Gerrit account", email)
+        return None
+
+    aid_dict = gout[0]
+    logger.debug("Email: '%s' has account-id: '%s'", email, aid_dict["_account_id"])
+
+    return aid_dict["_account_id"]
+
+
+def gerrit_lookup_account_ids(gerrit_settings, users):
+    """
+    Returns a list of account-ids given a list of user tuples
+    """
+
+    account_ids = []
+
+    for user in users:
+        aid = gerrit_find_account_id(gerrit_settings, *user)
+
+        if aid:
+            account_ids.append(aid)
+
+    return account_ids
+
+
+def gerrit_add_accounts_to_group(gerrit_settings, groupname, account_ids):
+    """
+    Given a group name and a list of account-ids, add accounts from group
+    """
+
+    gid = gerrit_group_id_by_name(gerrit_settings, groupname)
+
+    # https://gerrit-review.googlesource.com/Documentation/rest-api-groups.html#_add_group_members
+    members = {"members": account_ids}
+
+    gurl = "%s/groups/%s/members.add" % (gerrit_settings["url"], gid)
+    gout = json_api_get(gurl, gerrit_headers, members, True)
+
+    logger.debug("output of adding accounts to group: %s", gout)
+
+
+def gerrit_remove_accounts_from_group(gerrit_settings, groupname, account_ids):
+    """
+    Given a group name and a list of account-ids, remove accounts from group
+    """
+
+    gid = gerrit_group_id_by_name(gerrit_settings, groupname)
+
+    # https://gerrit-review.googlesource.com/Documentation/rest-api-groups.html#_add_group_members
+    members = {"members": account_ids}
+
+    gurl = "%s/groups/%s/members.delete" % (gerrit_settings["url"], gid)
+    gout = json_api_get(gurl, gerrit_headers, members, True, True)
+
+    logger.debug("output of removing accounts to group: %s", gout)
+
+
+def sync_group(settings, groupname):
+    """
+    sync a group from Crowd to Gerrit
+    """
+
+    cusers = crowd_users_list(settings["crowd"], groupname)
+
+    gusers = gerrit_users_list(settings["gerrit"], groupname)
+
+    logger.info("Crowd users: %s" % cusers)
+    logger.info("Gerrit users: %s" % gusers)
+
+    # lists of users to add or remove from gerrit
+    add_g = []
+    rem_g = list(gusers.keys())  # default to removing all users
+
+    # go through list of crowd users by email
+    for cemail, cuserid in cusers.items():
+        # if a user is found, remove from rem_g list
+        if cemail in gusers:
+            rem_g.remove(cemail)
+        else:
+            add_g.append((cemail, cuserid))
+
+    logger.info("Users to add: %s" % add_g)
+    logger.info("Users to remove: %s" % rem_g)
+
+    add_aids = gerrit_lookup_account_ids(settings["gerrit"], add_g)
+
+    logger.info("Account-ids to add: %s" % add_aids)
+
+    gerrit_aids = [gusers[e]["_account_id"] for e in gusers.keys()]
+
+    logger.info("All Gerrit account-ids: %s" % gerrit_aids)
+
+    aa_nodupes = [a for a in add_aids if a not in gerrit_aids]
+
+    logger.info("Duplicates removed: %s" % aa_nodupes)
+
+    if aa_nodupes:
+        gerrit_add_accounts_to_group(settings["gerrit"], groupname, aa_nodupes)
+
+    remove_aids = [gusers[e]["_account_id"] for e in rem_g]
+
+    # user with a different primary email could be in both add and remove lists
+    rem_filtered = [a for a in remove_aids if a not in add_aids]
+
+    logger.info("Account-ids to remove: %s" % rem_filtered)
+
+    if rem_filtered:
+        gerrit_remove_accounts_from_group(settings["gerrit"], groupname, rem_filtered)
+
+
+# main function that calls other functions
+if __name__ == "__main__":
+
+    args = parse_crowdgroups_args()
+
+    # only print log messages if debugging
+    if args.debug:
+        logger.setLevel(logging.DEBUG)
+    else:
+        logger.setLevel(logging.CRITICAL)
+
+    # load settings from JSON file
+    settings = json.loads(args.settings.read())
+
+    # global, so this isn't run multiple times
+    gerrit_headers = [
+        basic_auth_header(
+            settings["gerrit"]["username"], settings["gerrit"]["password"]
+        ),
+    ]
+
+    # sync each group
+    for groupname in settings["groups"]:
+        sync_group(settings, groupname)