Change to add_group_users function as didn't seem to work.

[WestonSF]

Changed:
user_names = _unpack(user_names, 'username')
postdata = self._postdata()
postdata['users'] = ','.join(user_names)

To:
postdata = self._postdata()
postdata['users'] = user_names

[brianjbaldwin]

Thanks for the commit, this was non-working and this small edit fixes add_group_users