the beta is open for all those who preordered entrenchment. so, if you haven't bought it yet, you won't be able to join the beta yet (you still can, once you opt for the pre order). I'd say it's a good system to ensure that people who participate have a stake in the outcome and participate. give feedback, etc. and do not just want to get to try new stuff.
you can pre-order it via stardock's impulse, it's costs a whopping (attention: sarcasm) $9.95. for this micro expansion alone, there will not be a retail version. not worth the materiel and logistics cost.