By the way, if you know the handle of a community, e.g. fediverse@piefed.social for http://piefed.social/c/fediverse, you can add a ! to the beginning of it, e.g. !fediverse@piefed.social, and send it as a DM to an alt account, or as the text body of a post to some test community. Then when clicking on the hyperlink it creates, initial federation with that community should start and let you subscribe. In the example I gave, it becomes !fediverse@piefed.social. It works also with Peertube channels and Mbin magazines, and I'd imagine so too but yet to test, with Friendica groups and Wordpress blogs.