[quote=Mahz]
But there's a big difference between a public group conversation and a private conversation where the creator declares the participants ahead of time.
[/quote]

Mahz is right. At this point, there's no way to add or remove recipients to a private conversation, so you're effectively stuck with the participants you started. That works if you have a set gang of lounge-goers, but if you're open to anyone, that kind of makes it difficult for newcomers without starting new conversations all the time.