Groups that can only hit certain groups?

Hey. I am trying to setup a cool event type thing where there is 2 teams. The teams are setup via permissions and I want to make it so players in 1 team can only hit the players in the other team, vise versa. (Friendly Fire). Anyways, any idea how to do this without adding a major plugin that will change what I already have. I just want a simple permissions based thing