Hi BVS,
You can accomplish this by using Tags from within Method.
First, take note of the customers that you would like to email. Then go to the Customers > Customers and Contacts List screen, search for each of the customers, and click the Go To... link to the left of their name. When in the Edit Customer screen, scroll to the bottom and select the Contacts tab. Select the main contact from the list here and click on the Edit Contact... link to the left of their name. In the top left of the screen that pops up you should see a tag icon. Hover over it and in the box that appears enter a name that you will use to identify this group of customers that you will be emailing, then click the Create button. Hover over the tag icon once more and check the box to the left of the tag you've just created, then click the Save & Close button. Repeat this process for the rest of the customers you want to email (for these customers just check the box to the left of the tag, you will not have to create the tag again). The last step would be to go back to List Builder, click on the Fill list with Contacts... button, click on the checkbox to the left of Include contacts... under the "Tags" section and then click on the checkbox to the left of the tag that you created earlier. Click the Insert Contacts button and proceed with List Builder as you normally would.
Now, if you wanted to email a group of customers with invoices due on a certain date, that would be easier. You can do this through the Due date from: and Due date to: fields under "Fill with Invoices" in Step 1: Fill your list of List Builder.
I realize that the above solution may be rather cumbersome if you have a lengthy list of customers you wish to email. There is a way to email all of them using the Import/Export tool to export customers, update in bulk, and then import them back into Method, however it's a bit tricky to do this so it's only worth it if you've got a lot of customers you have to update. If you want to go this route, let us know and we can help you get to it.
Hope this helps.
- Justin