Quicksand Fans

Does Fetlife have any groups like this one? In fact, does anyone have experience using the site? I heard people have found locals to sink with through it.

You'll want to look up the group 'Deep Sinking', which has over 600 members.
There is also a smaller group called 'quick sand/mud /deep sinking'.
Both are the only relatively active groups. All others are pretty dead, seldomly used or abandoned. Very rare niche crowd on FetLife. Honestly not really worth it (dialogue doesn't really change too much, having been on FetLife myself for 10+ years...at least that I have seen).

Sarge is pretty much correct about Fetlife in general. I think, however, that every group is different. Deepsinking is not very active but it is up to members to generate activity. Something I personally perceive as a problem with Fetlife itself, and also with Discord, is the tendency many people have to dilute things by starting more and more groups with more and more narrowly-defined purposes. That is, I think, a function of the way those systems are set up. There is also a quicksand group on Reddit.