I agree with Shadow about mapping the field. I play with about twenty guys. Four go out with radios and do sniper/early warning stuff. Then we split into 4-man rifle squads and each team picks a part of the field to control. But that's only with wide-area ball.
If the field is long and narrow, we stack up about three-quarters on one side and flank silently for the rear. The other quarter makes lots of noise and gets the enemy's attention. Ultimately, we try to lock them out of their spawn. It frustrates people royally, but we still win the match.