Comment 6 for bug 1538169

the root cause is:
in instance_group object, a member is add to DB but not add in instance_group object
members = db.instance_group_members_add(context, group_uuid,
                instance_uuids)
https://github.com/openstack/nova/blob/master/nova/objects/instance_group.py#L196

so in compute.api, the resource count won't increased.
                        count = objects.Quotas.count(context,
                                             'server_group_members',
                                             instance_group,
                                             context.user_id)
https://github.com/openstack/nova/blob/master/nova/compute/api.py#L935