If you're on Mastodon and know the web address of someone on the Fediverse, you can use this to follow them.
1. Copy their profile's web address into the Mastodon search box (see screenshots).
2. Click search, this should bring up a link to the profile and a follow button next to it.
3. Click follow!
This works with many kinds of Fediverse addresses including PixelFed, PeerTube and others. Try it!
You can also use this technique with web addresses of individual posts.
mastodon.at is a microblogging site that federates with most instances on the Fediverse.