I saw the list on here the other day of 400 slack groups. I scanned through and mostly were very small or niche. But I ended up joining 3 new ones that look good with big populations and so far lots of intelligent non-spammy talk. I've also shown a couple I'm already a member of which I recommend...
techlondon (as per the name I guess it's mainly for london/uk)
growth marketing pros (was already member)
founders network (was already member; application only, but worth it; UK members only I think)
I hate linkedin groups as mostly spam. but I found this yesterday and looks interesting for cofounder/idea finding, Ideas Seeking Co Founder
See you in there and say hello ;-)