This is a Group MMS conversation, but the given participant has neither an Address nor a ProjectedAddress.
Possible Causes
You may have inadvertently provided an Identity (chat user), rather than an Address (mobile phone).
You may have mistyped or inadvertently left out the ProjectedAddress parameter.
Possible Solutions
Provide a Projected Address when adding this Participant to the Group Conversation. This is the correct path when adding Chat participants to a Group MMS conversation.