There is no way to arbietrarily visit someone else's island, you have to be invited somehow.
If you don't have friends with a Switch, you can visit someone's islands with a "Dodo code" (i.e. a unique-ID for that island). As someone mentioned, some folks have gone via Twitter and I magine Reddit has a subreddit for it too.
One good thing is unless you are "best friends" on the game (this is a an additional step to being Switch friends), they cannot do many things on the island (chop down trees is the main thing I noticed).
