MulticastGroup is a published property in TIdIPMCastServer that identifies the dotted-decimal representation of the multicast group address for the server. 

MulticastGroup is used when setting socket options for a binding used by the multicast server, and when transmitting datagram packets to the network interface. 

MulticastGroup must contain an IP address acceptable to the IsValidMulticastGroup method, or an exception can be raised when creating, accessing, or releasing the Binding used by the multicast server.

TIdIPMCastBase.IsValidMulticastGroup Binding Send