You only need to add people to a team if you want others to be able to edit the island with you or play a private/unpublished code because you want feedback on your island.
To play the island privately you go through the publishing workflow, but make your island Unlisted, which means it won’t show in Discover.
An Unlisted publish provides you with a shareable link code that anyone you share the code with can use to play your island. It’s like a YouTube video where you can hit the URL but it won’t show up in search (Discover in our case).