You can add, remove, and manage users from your Settings in Portal. Click More on the left Navigation menu, and then Settings. Scroll down to the Users section.
User Permissions are available on the Premium Plan and higher. To adjust User Permissions for a User on your account, click the three dot (...) menu on that user in the User section in Settings:
User Permissions are setup in easy-to-use Groups, so you can assign Users to a group for "standard" permission sets.
Alternatively, you can create unique permissions for each user by clicking "Permissions" in the User menu (above).