Invite a Customer to their Workspace in my Portal

You must be an Account Level Moderator to Invite Customers to their respective Workspace.
First, navigate into the Workspace belonging to the Customer, which you need to Invite them to.
Click on the Share icon near the upper-right that looks like 3 stacked silhouettes.
Enter the email address for the Customer, change the permission level from the default 'Downloader' to 'Creator'.
Click the 'Invite' button.
Click the grey X at the top-right of the 'Invite Users' dialog.