You can find accs by typing in the unique ID that Mangago gives all their users but you can't find them by username. Mangago also doesn't have a feature where you can just simply look up the username of someone. Your best bet is looking through the comment sections of mangas and posts.
Example link:
[http://www.mangago.me/home/people/570012/home/...... reply
What you can also do is type in google ‘Mangago (user)’ and you may be able to find it. If you are in contact with this user, you can also ask them to send their URL to you. reply
