Updated 03-20-2022 08:52 PM
Get someone from all 5 colors in the Hot Tub at the same time, the best way to do this is play an instrument in the hot tub. I have tried asking people to join with pet names but they will refuse. Arena will be deleted very soon, so i reccommend doing this as soon as possible.
Edit: this achievement is currently unobtainable (most likely permanently) as arena was removed from Sea of Thieves.