you can search for !communityName and your server will look it up and you can join it. No need to have multiple accounts.
Edit: instead of the above you can also search for the link to a community on a foreign server to look it up, both works.
i.e. if you were on a server that didn't yet know about https://beehaw.org/c/news, you could search for https://beehaw.org/c/news or !news on a lemmy server so the lemmy server will recognize that community and list it, you'll then be able to join it, participate, mod it (if you're given the rights) etc.