Handle added audio group member.

def multizone_member_added(self, member_uuid):
        """Handle added audio group member."""
        casts = self._casts
        if member_uuid not in casts:
            casts[member_uuid] = {'listeners': [],
                                  'groups': set()}
        casts[member_uuid]['groups'].add(self._group_uuid)
        for listener in list(casts[member_uuid]['listeners']):
            listener.added_to_multizone(self._group_uuid)