OnePlus has always been quite active with the community, looking for feedback from users to help improve future updates of the OxygenOS build and make sure users get the best experience possible. The OnePlus 6 has been no exception, even before it was released, but now OnePlus is opening up a new closed beta for users to give more direct feedback to the company.
In essence this means there will be three versions of OxygenOS floating around on user devices. There's the final stable updates which go to all OnePlus 6 owners, the open beta which lets users test future versions of the software ahead of time, and now the closed beta which is said to be as close to a pure developer build as you can get.
There are some catches, naturally. The first one is that you need to own a OnePlus 6 device, which is obvious given the nature of the software. You also need to be an active member of the OnePlus community, and be willing to "constantly communicate and offer feedback" for the development team via Slack. That's not very strict criteria, but the key caveat here is that there are only 100 places available. So getting in is going to be quite difficult.
On top of these you have to be willing to constantly "clean flash" your phone, and potentially lose some data in the process, and sign a non-disclosure agreement that swears you to secrecy. That means no disclosing information about upcoming features with anyone that isn't already working for OnePlus or part of the beta.