====== onJoinGroup() ======
wrtc.onJoinGroup() is called when we have received an invitation to a group call.

You do not need to write any wrtc.onJoinGroup() function in order to make group calls - only if you want to make limitations to the automatic group call sequence.

===== Argument =====
wrtc.onJoinGroup(arg):
  * arg - JSON structure: {type:'joingroup', gid:…, from:…, from_dn:…, members:…, bridge_addr:…, bridge_dname:…}
    * gid - group ID
    * from - address of client starting group call
    * from_dn - display name of user starting group call
    * members - number of members in group
    * bridge_addr - address of voice bridge hosting the group
    * bridge_dname - display name of voice bridge hosting the group