Great question! If you go to Clients > List in the top left, there is a filter icon. If you click on that icon, you can choose to filter your client list under the "Tagged with" section. When you select a tag or multiple tags, the clients will show in your list. You can select all of the clients, and click the "Email" button to send a mass email to these specific clients.
You can learn more about how to send mass emails in Full Slate here.