When I signed up, I got 2 invites. They let us know that invites increase as we are more active. However, I only looked around, joined a room twice, and my invite capacity increased.
It will eventually make its way to everyone, but the way they released it is smart.
They are going an MVP route while scaling their platform as they go in collaboration from the feedback of users deep in the platform.
Saves them money, energy, & other resources in the long run without having to spend any extra dollars on marketing since its exclusive to word of mouth on one type of device.
If you are on the fence, you will eventually be allowed on the platform. I imagine that's the case. The benefit of getting in now is first movers advantage if you are looking to build a platform of some sort.
I see it becoming a accessible, universal, discussion-based linkedin (in so far as how we use comparisons to commonly refer to innovation).