The beta of
The Banner Saga Factions was already
open to all backers, but is now also
open for non-backers of the Kickstarter campaign, if you pay for it.
If you mised the Kickstarter campaign you can still join for $15. You apparently get a steam key for Factions, you can play now, and for the complete game, which will take some time before you can play it.
More information.