Reach Campaign matchmaking next month
Halo Reach Campaign matchmaking may not be available right now, but Bungie's promised that's all about to change: it's being added in early October.
Bungie wasn't happy to include Campaign matchmaking up to now, mainly because it was worried people would see the ending before they'd even started the Campaign proper, as explained in Bungie's weekly update.
Although spoiling the game will still be a possibility, Campaign matchmaking will offer all players in the party a vote as to which mission they’ll end up playing, so if you don't want to play in a mission you haven't reached in the single player Campaign, you can always opt out.
Bungie says the Campaign matchmaking update for Reach will occur in early next month.