I don't believe this is possible. I could not even find a way to send an email manually after a Livetrack has started. I think there could be multiple applications, like messaging new contacts at a later point or messaging external people when your group (you need to enable Livetrack for Grouptrack) reached the real starting point, but the current implementation seems just too primitive for that.
To be more precise, you can actually copy the Livetrack link by clicking the share icon in Garmin Connect app and then share the link at any point through any means with any followers (including emailing them as you finish your ride). It's just that I don't think you can trigger an automatic email to your regular followers, even manually.
Also note that if you want people to see you ride after you have completed it, you need to extend the visibility of your Livetrack in settings.
