def add_group_user(self, group_id, user_id):
data = {<EOL>"<STR_LIT:id>": user_id<EOL>}<EOL>response = self._perform_request(<EOL>url='<STR_LIT>' % group_id,<EOL>method='<STR_LIT:POST>',<EOL>data=json.dumps(data))<EOL>return response<EOL>
Adds an existing user to a group.

:param      group_id: The unique ID of the group.
:type       group_id: ``str``

:param      user_id: The unique ID of the user.
:type       user_id: ``str``