Hello,
You can use meetme conference application for this, hold the call, transfer to the MeetMe room and join MeetMe room, simple....
For example, A calls B, B wants to invite someone else in the conversation, B will transfer the A\'s call in XXXX room and dial B dials C and do the same (tranfer in XXXX room) and then B joins the MeetMe room.
Regards,
Babar Shafiq Nazmi.