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).