That's would probably be the best circle jerk in the world, but you've got send everyone underage out of the room to play Minecraft together or some shit
That was literally the first word into my head. I don't think I would actually do that, though. Honestly I'd probably just want to leave, I seem annoying.
5th grader: Troll him with my femboy outfit while doing a Fortnite dance on him. I was really homophobic at that time.
10th grader: Tell him to just buy that thigh high socks and not get sucked into 8chan that much.
4th semester: Pls use noise cancelling earplugs to that one concert.
Overall I would tell about how I got to my current state and what happened generally. It would be interesting how each of the versions of myselves would react.
I need more details, dude. If I hung out with them for a year, would a new me appear or would we all grow older together? In the case of the latter, would a new 1 year old me appear when the previous 1 year old me turned 2? These are important considerations!
I assume this is a multiverse situation, and at the end of the party they go back into their normal timelines, only with the experience of that gathering now with them.
No changing the past for yourself, but maybe changing the course of all of theirs based on how they react. With that in mind, I can't think of a single thing I'd say that would help any of them, except maybe:
"Let's all work on figuring out how this happened, because this rather breaks the universe, doesn't it?"
Don't know what I'd do with them, but my current self and 20 year old self wouldn't get along very well (I'm old). Probably have to divide them in two groups and keep them apart.