Sometimes you can ask in zone chat and get a group together. During prime time you can also sit in a zone where you need one of the bosses and join in when a group comes along to kill it. Get in one shot and you will get credit for killing the boss. On one character last time we had this event I had one boss left to kill. I waited around and watched groups from the other faction kill it a few times. A group from my faction never showed. Finally I waited for an opportune moment rushed in while the enemy was killing the boss threw out an AoE and got a hit in before I died. Laid there until they finished the fight then moved on my happy way. Remember to unload telvar stones though if you go that route.

If you are in a guild they often have events based on these festivals to help members get achievements. If you are not in a guild you really should try and join one. I didn't have any friends play ESO either but joined a guild and now I consider many people in the guild true friends.