diff --git a/zulip/group_membership.py b/zulip/group_membership.py
new file mode 100644
index 0000000..d89e73d
--- /dev/null
+++ b/zulip/group_membership.py
@@ -0,0 +1,117 @@
+
+import gevent.monkey
+gevent.monkey.patch_all()
+
+import logging
+
+import requests
+
+logging.basicConfig(level='INFO')
+
+class Client(object):
+	def __init__(self, base_url, api_key):
+		self.base_url = base_url
+		self.api_key = api_key
+		self.session = requests.Session()
+
+	def request(self, method, *path, **params):
+		# TODO api key
+		if method == 'GET':
+			args = {"params": params}
+		else:
+			args = {"data": params}
+		url = "/".join([self.base_url, "api/v1"] + list(path))
+		resp = session.request(method, url, **args)
+		resp.raise_for_status()
+		return resp.json()
+
+def get_membership(client):
+	"""Returns {group id: member set}"""
+	return {
+		group["id"]: set(group["members"])
+		for group in client.request("GET", "user_groups")["user_groups"]
+	}
+
+def update_members(client, group_id, old_members, new_members):
+	added = new_members - old_members
+	removed = old_members - new_members
+	client.request("POST", "user_groups", group_id, "members", add=list(added), delete=list(removed))
+
+def determine_members(schedules, hour):
+	return set(
+		user for user, schedule in schedules.items()
+		if hour in schedule
+	)
+
+def run_hour(client, user_map, groups, hour):
+	logging.info("Setting groups for hour {}".format(hour))
+	members = get_membership(client)
+	def run_group(group_id, schedules):
+		new_members = determine_members(schedules, hour)
+		new_members = set(user_map[id] for id in new_members)
+		assert group_id in members, "group {} doesn't exist".format(group_id)
+		update_members(client, group_id, members[group_id], new_members)
+	gevent.pool.Group().map(run_group, groups.items())
+
+def parse_config(conf_file):
+	MAX_DAYS=8
+	with open(conf_file) as f:
+		config = yaml.safe_load(f)
+	assert "url" in config
+	assert "api_key" in config
+	assert "start_time" in config
+	all_users = set()
+	for group_id, schedules in config["groups"]:
+		all_users |= set(schedules.keys())
+		for user_id, schedule in schedules:
+			hours = set()
+			for part in scuedule.split(","):
+				part = part.strip()
+				if part.endswith("/24"):
+					hour = int(part.split('/')[0])
+					hours |= set(24 * day + hour for day in range(MAX_DAYS))
+				elif part in "ZDAN":
+					shift = "ZDAN".index(part)
+					hours |= set(hour for hour in range(24 * MAX_DAYS) if (hour % 24) // 6 == shift)
+				else:
+					hours.add(int(part))
+			schedules[user_id] = hours
+	missing = all_users - set(config["members"])
+	assert not missing, "missing: {}".format(", ".join(missing))
+				
+
+def main(conf_file, hour=-1):
+	"""
+	config:
+		url: the base url of the instance
+		api_key: authentication
+		start_time: Time of the first hour, as epoch int
+		members:
+			NAME: USER_ID
+		groups:
+			GROUP_ID:
+				NAME: SCHEDULE
+	Where:
+		schedule: Comma-seperated list of hours
+		hour: One of:
+			Integer hour, representing that hour of the run
+			N/24, expanding to hour N of each day (eg. 0/24 is midnight each day)
+			Z | D | A | N, expanding to all hours of Zeta, Dawn Guard, Alpha Flight, Night Watch respectively.
+	"""
+	config = parse_config(conf_file)
+	client = Client(config["url"], config["api_key"])
+	user_map = config["members"]
+	groups = config["groups"]
+	if hour >= 0:
+		run_hour(client, user_map, groups, hour)
+		return
+	while True:
+		hour = (time.time() - start_time) // 3600
+		run_hour(client, user_map, groups, hour)
+		next_hour = start_time + 3600 * (hour + 1)
+		remaining = next_hour - time.time()
+		if remaining > 0:
+			time.sleep(remaining)
+
+if __name__ == '__main__':
+	argh.dispatch_command(main)