[syadegari]

While not a community itself, Terence Tao's blog (https://terrytao.wordpress.com/) is a good source of high quality, clearly explained maths. As you may know, he has a very broad range of interests, and there might be something for you to pick up and get involved in. He has also been involved in several collaborative math communities, which he writes about at length in his blog. I recall Polymath project and recently some project involving Lean Theorem Prover that even showed up on the first page of the HN.