In Sololearn, you don't really "add friends", clearly this doesn't work like the popular social media platforms. All you meet here are like- minded people having an ardent interest in programming working on a variety of languages.
If you want to interact with your peers, there isn't any private chat option, instead, you have to post it on a platform that envitably has public accessibility.
If you are interested with the developments associated with a certain user in Sololearn, you have an option to follow them.
Plus, there aren't any barriers in making someone as a friend, as far as my experience goes here,so sharing your knowledge and getting inspired by someone's work is graciously welcome here.