@sjml can you get to their user page on the instance, there should be 'remote follow' button note that you have to put your own username and instance in the form there without the first @ ie.
@sjml https://www.youtube.com/watch?v=t1lYU5CA40o
Just like GnuSocial, Mastodon is a federated network, in fact gnusocial and mastodon nodes can even federate with each other. If you visit someon on another site running gnusocial or mastodon and you click the remote follow button, you can input your handle ([email protected]) to the other site and it will link with your server allowing you to follow your freinds across websites.
@sjml you can put eg.
in the search bar to find users on other instances. I'm pretty sure you need to know the exact name you're looking for, though.