User registration can be done in 3 main ways.
1. By Invitation. In this case an already registered user (with right privileges) will create you an account. Soon after an email should arrive at your address with a link to click and login.
After clicking New User another window will be shown to fill required information like: Name, Username, Email, Access Level, Avatar ect. At the end click Create User.
2. By Federation Authentication. This is only possible if you have an account provided by an Identity Provider on which GitLab has permission to ask authenticating users.
3. By requesting access for collaborator from outside GN Project. See How to invite people from outside GN4-3 project guide.