I don't really understand what your asking, so i'll go through how to play a matchmaking game step by step.
1.From the Halo 3 main menu, sign in to your gamertag (you have to have a current gold subscription)
2. Choose matchmaking (the second item down, right under campaign
3. Choose what playlist you want. (Team Slayer, etc.)
4. Choose start game.
5. Wait for other players to join and the game will start
If you are asking about playing online in campaign, you can only play online with people on your friends list who you invite to a game.