I signed-up on January 16, 2015 to try to get an invitation to join the service, before they opened to the public. I got an invitation on Friday, February 6, 2015, and I started my seven day free trial.
Previously, I purchased season one of Star Wars Rebels via iTunes to be able to watch the series. With my season pass "Call to Action" will not show up in my iTunes feed until tomorrow; however, I was able to watch the episode featuring Tarkin, over the weekend with my Sling TV app.
I have included two screen shots from my iPhone. It was nice to be able to watch this episode early, like so many other people have been doing all season long.
As of today, it seems that Sling TV is now open to the public to enjoy. For more info check out Sling.com.