Justin took to his Instagram to share a group shot of the band posing together at the party.
“JC‘s 40th… And, if you don’t know now you know…” he captioned the image.
Earlier in the day, the other guys shared their birthday wishes with Lance writing, “Happy Birthday #JCChasez!!!!” and Chris adding, “Happy birthday @JCChasez ! Welcome to the old man club!”
Also in attendance at the party were Chace Crawford, director Bryan Singer, producer Dallas Austin, 5 Seconds of Summer bandmate Luke Hemmings, Brody Jenner, Darren Criss and Tara Reid, who all enjoyed music from beeFOWL.