If you're just looking to hang around and score PBs to surf and turf(war) with friends, Discord is probably the place to do it. If you're looking to git gud and seriously join a team, Discord is also probably the best place for that.
The r/Splatoon discord(linked in my signature) has regular users and a Match Search channel for non-commital grouping, but it's not the most active. Where I mostly see things going down is on team servers that are either really big, really casual, or both.
Since the r/Splatoon discord doesn't allow posting links to servers, it's a bit of a hassle, but you can ask people to PM you their team servers or other servers they know of that might fit what you're looking for.
For Starters, here's the Free Agency server:
https://discord.gg/Ny5v8X6 This is a server specifically for recruiting team members/looking for teams. I don't want to hand out invites to otherwise private team servers, so from there it's just a matter of getting know people and being like "hey can I join your server?" and bam you've got yourself a neat little community to hang out with.
Good Luck friend.