The Bulk Actions menu on the Contact List in your Merchant Center allows you to select multiple customers and perform an action, including subscribe, unsubscribe, or send a specific email to a group of contacts.
Next to a contact, checkboxes can be selected for each contact on which you would like to perform an action. You can also click the checkbox in the title menu to select all contacts on a page. If you want to select all of your customer contacts, click "Select all [xxxx] customer contacts" in the message that appears above the title menu.
Once the desired contacts have been checked, click the gray "Actions" drop-down menu and select your desired action.