I had some problems figuring out how to get a custom game in the beta because all of the videos were just videos of the custom game lobby. http://www.youtube.com/watch?v=GXkGxz_WIvw.
1.Anyway you need to tell everyone in the party to send you an invite if they are removed from the party, and to not change any of the settings when he is removed.
2.Get 7-9 people in a party (including yourself) .
3. Tell one of the members of the party to go into matchmaking.
4. Tell the person in the matchmaking lobby to send you a party invite.
5.Accept the invite and Bring the party.
It is likely that one of the players will be removed. but they dont always get removed.
6.After the removed player sends you an invite you should bring your party to his party.
When you join the party it will be custom game, or you did something wrong.
7. Some one will not be removed every time. If no one is removed then you need to redo all of these steps.
8.Once you are in the party you can play whatever they are playing, or press B. Quit the party. and you will still be in a custom game lobby.
9.Then have fun and share the fun of the custom game lobby by having the people in your party do step 8.