1. Log into your Merchant Center and navigate to the "Contacts" tab
2. Click the gray "Manage Lists" drop-down menu in the top right-hand corner and select the list you would like to add your contact to
3. Type the contact's name or email address into the search bar and click on their name in the drop-down menu that appears
To add a contact to more than one list, repeat this process for each of the lists you would like them added to. A contact can be added to up to 5 different lists.