You need to get a win in each of the seven MP game modes, there are very few people playing any mode except Team Deathmatch so it's probably one you'll need to boost.
The seven modes are:
Free For All - You need to win one ten minute round - can't be any quicker
Team Deathmatch - Best of three ten minute rounds - Can't be any quicker
Data Capture - One ten minute round - Can't be any quicker
Domain Control - Best of five rounds - Quickest a round can be is three minutes.
Demolition - Best of five rounds - no respawns so game is over if you arrange to meet in the map and one team wipes out the others.
Team Survivor - Best out of five rounds. No respawns so you can meet up to arrange one team to kill the other team.
Operation - Best out of five rounds. No respawns so again you can meet up to arrange one team to kill the other team.
Just simply go into create match - set rule to change the mode. You can then invite people into your lobby. You need six people to start a game with maximum game size of ten people. You do have the option to kick any randoms that join your game.
As noted below, it needs to be done in a public match.