We're talking about extended free trials, which are available for a limited time right now at Groupon. These are obviously "selling fast", but at the time of this writing, you can still choose one of three free premium music streaming options for 90 days. Or rather two Premium options and a Plus service tier that's normally cheaper, at $4.99 a month.
Last but not least, Groupon will allow you to sign up for three months of Pandora Premium Family access at absolutely no charge as well. That's a savings of around 45 bucks, including premium music streaming rights for up to six different users.
As you can imagine, these promotions are only available for new subscribers who haven't used a free trial pass before, requiring coupon code redemption directly at Pandora, as well as credit card registration. Keep in mind that you'll naturally be charged the regular subscription fees once the first three months are up unless you cancel the service earlier, which is allowed but not exactly encouraged by Pandora.