You need to visit the community via your instance, not directly via theirs.
You can paste the full URL for their community into the search box in your instance and search for it, and you'll get a link to their community that you can click to accomplish this; you can subscribe to it from there, as well.
I don't know what community you're referring to specifically, but you'll end up with a URL that's something like http://sh.itjust.works/c/[email protected].