1) Join a friends player match
2) Get him to search for a ground war game
3) Make sure you join a pre-game lobby (13 people need to be in lobby)
4) Leave the lobby without party
5) Back out of the player match
6) Go to Local, Press (y) on system link and press (X) to join your friends session again
7) You should hear the voices of the pre game lobby but still be in the multiplayer menu.
8 ) Press (A) and go into system link
9) Create a class, choose a perk set and press (X) until it gives the option for pro perks
10) Buy and equip the perk you want!