You don't have to be on a particular instance/server to join a group. The groups work across all the servers :)
Just follow the group's account to start receiving messages from the group, and mention the group's address in a post to take part in the discussion.
If you put this address into your search box on Mastodon it will make the group's profile appear. Just click follow to follow it.
@ longcovid @ a.gup.pe
(remove the spaces first though)