A new user can only be created by an user with admin rights in the account.
Admins have access to the Suite-Manager and the Usersettings.
In the user-settings (visible in the left grey area) you can click on +Add to add a user.
The checkbox "admin" sets admin-rights for the user, which has then access to the Suite-Manager.
When you filled in all data for the new user, you can click the save button and continue to the user-rights. Afterwards set the rights, which the user should have access to and click save again.
Afterwards the user can login with the data you set in the user-settings.