Can not invite friends while playing NFS rivals in private mode

I Can not invite friends while playing NFS rivals in private mode.

But when my game mode is either public or friends I can do that easily from within the game itself via invite or form outside the game ( from the chat menu)

When I switch the game to private. I can not find the invite button within the game. from the chat menu it is dimmed !!!!

Very strange.

Any advise