inside the game, people sometimes make different clubs using saves. They are run unofficially by whoever makes the save, and that works as clubs for most. It's probably hard to find individual ones, just search "club" or "group", or just browse the recent saves until you see something.

We also have a groups section to do this: https://powdertoy.co.uk/Groups/Page/Groups.html
note that it's outdated, almost all / maybe even all of them are inactive, and there are some glitches because it uses the old look of the forums and was never updated. Thc change in look actually added a few bugs to it, so it's hard to post sometimes.