I nearly got one last week (but in the end didn't), and registered, and signed up for their newsletter, and since then, in about 5 days, have had two 24 hr. invites.
Therefore I would recommend to anyone seeking an invitation, to do the same.
That's what I thought at the beginning, but then I tried oxygenOS and for my taste is better... a very clean android and very stable and fast. It looks like a nexus.
But they are very slow with the updates, still 5.0.2
I read that they are bringing an update soon. I hope so... cyanogen is not supporting the OPO anymore because they broke relationships... but cyanogen still sent some updates. What I didn't know is that they already sent the 5.1.1