I'm still fairly "early" in the game, being only level 25, but I've yet to actually need a group, for, well, anything. The really big events that happen, people just naturally flock to, because that's a good chunk of XP, and they're also legitimately fun. Earlier today I was clearing out a cave of some centaurs, and a "kill the chieftan" event spawned right on top of me. I was running out of that cave system as fast as my legs could carry me, because there was no chance for me to come close to killing him. I get out of the cave with the guy hot on my heels, and there were about 20 players waiting for him, I didn't know any of them.
I've yet to see anything like a "raid" though. I think the game has them, but I'm not positive. Your mileage my vary, of course.