The problem is that XNA Game Studio doesn't allow you to be part of more than one game session at a time. While you could have the party leader collect everybody, and perhaps forward information about the session he chooses, there would have to be a lot of re-connect/dis-connect for this to work. You'd also have to have the host cooperating. Here's how I would see it working:
1) Party leader hosts "party formation" lobby.
2) When everybody is in the lobby, a random 32-bit number is generated (or a hash calculated from the gamertag of the leader).
3) Everybody disconnects, and the leader starts choosing a network session to join.
4) Once the leader joins, the network session gets re-published with one of the selection ints being the randomly generated int.
5) Meanwhile, all party members would be waiting for a session that contains that integer.
The experience for the others wanting to join that session would probably be bad, though, because I think you have to disconnect the current session to re-list it with new integer tags. In general, I think you don't quite have enough bits to make it work well using the current API. If you could host one session and join another session at the same time, then it would be simple, but sadly, that's not the case.
