Credit: Instagram Photo: Maks, Derek, Cheryl, Mark, Jack and Amber's (and Justin Bieber's) manager

Sometimes it seems like everyone in Hollywood knows each other — like it's one big high school with cliques and rivalries, but they all get together for homecoming and such.

Dancing With the Stars pro Derek Hough shared the photo at left over the weekend, writing, "Happy birthday, Jack! Onesi party, guess I only got the memo."

Season 17's Jack Osbourne just turned 28 on November 8, and you can see his pro partner Cheryl Burke in the pic, along with Derek; Derek's T-shirt twin Mark Ballas; Jack's lovely wife Lisa; a couple of girls we're not sure about; DWTS pro Maksim Chmerkovskiy (come back to us, Maks!); and, for whatever reason, Justin Bieber's manager Scooter Braun and Justin's bodyguard Kenny Hamilton.

Jack is the king of self-deprecating comments during DWTS rehearsals, and he makes Star Wars references that go over Cheryl’s head, but he’s not exactly The Everynerd. Earlier this season, he mentioned how he used to go dancing with Paris Hilton, and he does have a famous family with connections all over, so maybe it shouldn’t be too surprising that he’d end up with a major pop star entourage in his b-day pics. But why wasn’t the Biebs pictured? Maybe 'cause he's busy getting into shenanigans in South America?

Cheryl shared a more classic birthday photo (below) on November 8, with balloons and a cake showing their official DWTS promo photo. Someone also made Jack his own little mini Mirror Ball trophy, in case he and Cheryl don’t end up with the real one. It looks like the pic was taken in a rehearsal studio, and Cheryl tweeted, “@JackOsbourne is here rehearsing on his birthday! He never fails to impress me, I couldn't ask for a better partner!”

Happy belated birthday, Jack! Here’s what he and Cheryl will be dancing tonight on Week 9, (along with Derek and Amber’s styles and the rest of the Season 17 contestants, too).

Dancing With the Stars airs Mondays at 8 p.m. ET on ABC.

Credit: Instagram Photo: Cheryl and Jack on his birthday