in Flash Player 10.0 it's possible to connect three people in a chat. the publisher will have to send separate copies of the stream to each subscriber.
in Flash Player 10.1, for just two subscribers, the multicast spanning trees will typically settle such that the publisher is still sending separate copies to each of the two subscribers. multicast will be a win for bandwidth when the number of subscribers is above about 4 or so.
if you mean an N-way chat (N publishers and N subscribers simultaneously), then bandwidth-wise there's no difference* between N*N 1-1 streams and N multicast streams, since everybody has to get all the streams, that data has to come from somewhere, and the only "somewhere" is the other peers in the group.
* there is a bit of a difference -- P2P multicast has a higher overhead than 1-1 unicast streams, so an N-way chat will use more total mesh bandwidth with multicast than N*N 1-1 streams would.