Guitar Hero World Tour

From the Xbox Live menu change your mode to Band v Band then start Matchmaking. This online match needs to be completed, but does not need to be won.

It is rather hard to find seven other random people to make up two bands, so I would suggest having a band of four in your party already before you search. This seems to make it easier while trying to find an actual match. It is possible to bring in three other guest accounts to your Xbox Live menu if you have two guitars, a mic, and drums. Just make sure one of your guitars is selected to Guitar and the other is on Bass before you search (you can switch by pressing the Orange button).