Beat the game and save. Now start with the saved file. Go to Multipalayer and you'll notice that there're Mission Cooperation and Firefight Cooperation modes availible in Multiplayer.

Note: In Ghost Recon there are two different games: Ghost Recon and Ghost Recon: Desert Seige(DS) (used to be sold seperately.) After you beat the Ghost Recon (15 Missions), there will be 8 new missions(for Desert Seige).