Maybe I'm not using it correctly but I'm hiving issues using the search functions to find communities. The community i was trying to find was the Reptiles and amphibians community on mander (https://mander.xyz/c/herpetology). I'm not sure how to sub otherwise. Cheers!
With federation, if no-one has subscribed to it before you need to use the !community@instance format. This triggers the server to attempt to fetch the community for the first time. Otherwise you probably would have found it with reptiles (ithink)
To tag people you can also do @username@instance
When on the web interface it should try to auto-populate as well when doing those formats.