Well the easiest way, off the top of my head, would be to remove all the AI's weapons (just the guys you want chuckin' grenades), fill em up w/grenades and then have them dotarget and dofire.
To have them attack using grenades only, when they have other weapons, I'm not sure how you would do that. I don't know if you can specify what weapon you want the AI to use as they are pretty flaky with that stuff.
;D