Not sure if I'm answering your question, but you can see your list of joined communities here: http://com.nicovideo.jp/community I don't know how you can sort by activity, but you can check each page for their last broadcast date. I believe this list shows all the communities you have joined.
(this can be accessed by going to the nicovideo home page --> Other (そのほか) on the top bar --> Community (コミュニティ) --> go to the right corner to your Mypage menu and choose participating communities (登録中のコミュニティ)
Re: Leaving communities
I don't know how you can sort by activity, but you can check each page for their last broadcast date. I believe this list shows all the communities you have joined.
(this can be accessed by going to the nicovideo home page --> Other (そのほか) on the top bar --> Community (コミュニティ) --> go to the right corner to your Mypage menu and choose participating communities (登録中のコミュニティ)