final void listMembers(Message message) {
        String groupName = message.message;
        String user = message.token.getSubject();

        if(groupData.containsKey(groupName)) {
            GroupRecord gr = groupData.get(groupName);
            if(gr.owner.equals(user)) {

                StringBuilder sb = new StringBuilder();
                for(String member : gr.members) {
                    sb.append(member).append(',');
                }
                message.message = sb.toString();
                message.success = true;
            } else {
                message.message = "only the group owner may list members.";
                message.success = false;
            }
        } else {
            message.message = "group does not exist.";
            message.success = false;
        }
    }