Hm... don't quite know the answer to your question there. If you make a lobby in Upcoming Games, and someone RSVP's, you can use their gamertag to send them a game/party invite. Hope that helps!
Hey Brady, my name is Elliot. My group will be hosting many Flood lobbies soon. As you want to get into some custom game lobbies I'm guessing you would be interested. Click the link in my signature, all the info is there.