User permissions allow you to control what different users can access within the Web Portal and the App. You can only edit a User's permissions through the Web Portal, and must be an administrator or already have permissions to edit other User profiles.
In the Web Portal, select Admin > All Users > Edit arrow for the User (this will take you into the User's Profile where you can add and remove User Permissions) > Set Permissions > select Save at the bottom of the page.
