I was traveling near the edge of some big town (don't recall just which one) when I encountered some raiders. I engaged with one of them, but decided I didn't want to mess with the rest so I was continuing on my way. Suddenly, some scavengers appeared, and starting fighting the raiders. Every time a raider dropped I got XP.
Thinking that was cool, I hung around to watch (and collect whatever XP I could just 'cuz ). So, the scavengers won pretty quickly, and just as the battle was finishing up three supermutants showed up. I joined with the scavengers in fighting them and I got XP on all three of the supermutants too, though I only killed one. When the fighting was over, one of the scavengers leveled his weapon at me and basically told me to beat it. I decided to comply, though I looted a couple of the raiders on my way out.
All in all, a pretty satisfying encounter...