You can search for other Soundtrap users directly from your profile page (1) or from the collaborate tab inside the studio (2). Click "View" (3) in the search results to visit the profile page of the user you want to follow. Then click the Follow icon (4, 5) to follow that user. Done!
(1) Profile page - search field
(2) Studio - Collaborate tab - Find user
(3) Search results - View
(4) Profile page - Follow icon
(5) Follow icon