Right now, this is not an available feature through Tableau Server 8 subscriptions. Vote for it to become one here:
A workaround I discovered, and is recommended by Tableau via the knowledge base, is to download febootimail command line emailer and use tabcmd. There, you can specify to whom you want the email to go (beit a google group distribution list or a real email address) so long as you have a valid smtp server account to send mail from. Here's the kb article on how to script it. Once written, save the script as a batch file and automate it using windows task scheduler. If you have a bunch of dashboards you'd like to have to force subscriptions for, I'd recommend making sure you have the space on your Tableau server to create the PNGs or PDFs of the subscribed views during run and make sure to delete them on completion.