To add a curator to a studio you curate, you must be a manager of the studio. First, go to the "Curators" tab and press the button that says "Invite Curators". From there, click on a user from the bar at the bottom of the page that contains all the users you are following, or type the username in the box below the button. If they have already been invited or they are already a curator of the studio, then it will give you an error message. The user you have selected will get a notification asking them if they would like to curate that studio. You cannot invite someone twice.