I am trying to understand what you mean from what you said. :)

You say that when you Add all rooms, it groups all the rooms. Perfect. That is what you wanted it to do.

Then you say, you want it to NOT add a room. Well the obvious answer would be to not select the room you want to not be a part of the group when you add rooms.

Am I missing something? Are you asking if there is a way for the C4 system or the SONOS system to ignore a grouping request if a room is already in use? As far as I know, no. The thing between the ears of the person in control is the only thing that can make that happen. :)