shout box will help you out a lot. Especially promoting the event daily, and hourly on the day and every fifteen minutes on the last hour. People tend to be excited about something if you show excitement. Now on the off chance it doesn't work out exactly and you get more people messaging you over Xbox Live than RSVP, you can always have two captains choose people to have on their teams. Did this with one of my lobbies. A lot of fun.
@ Fuzzle, Your sig almost made me cry. I don't know why but stories like that are just awesome. I love bungie too and feel that their quest for world domination will succeed unlike blizzard who stole that from them and now sucks! I will never buy a Bliz game again but Bungie, They have my support.