Well, my purpose was to stop those annoying multichat window with too many people that maybe you don't even know. I suppose that if i'm a member of a Group is because i would like receive messages or something from those people, without confirmation on window open.
So if i open i single chat and then invite other people conversation warning is definitely not needed, as for groups, in my opinion.
Anyway, i understand that for now it's too hard to do what i'm asking... I really hope Patchou will find a solution, because a ChatWnd::IsGroupChat, as you said, could be really useful!
Thanks again for help!
New idea!
How about checking if contacts in multi-contact chat are in my contact list? For example, if contacts > 5 and most of the contacts are not in my list, it is probably an annoying multi-contact window, and not a group i subscribed to, so i will ask confirmation
/ 
