An advert can be assigned to one account and one account only. This is done when you create or edit the ad using the Advertiser option. Simply select the name of the user you want to allow to view and possibly edit this advert and save it. Important here is to enable click tracking otherwise he/she can’t see the performance!
Optionally you can enable advertisers to edit their adverts as well. You can enable this option from the AdRotate Settings.