OnePlus already released open beta for the OnePlus 6 back in May, and it keeps updating it frequently. Now the company has announced that it is opening registrations for closed beta program to get feedback and refine or revise the OxygenOS based on that before pushing the public beta.
The closed beta usually includes an elite crew of OnePlus community members who are not allowed to share the builds with others since they get new features weeks or even months before they are released to the public. The company also says that those who are selected for this are required to sign an NDA (Non-Disclosure Agreement) with OnePlus to be part of this program.
It will be selecting 100 people into this Closed Beta Program, and in order to be eligible the OnePlus 6 users need to be an active member of OnePlus Community and should be willing to constantly communicate and provide feedback to the OnePlus team on Slack
Those who are interested can register here. Last month OnePlus released the Android P Beta 3 (DP4) for the OnePlus 6 smartphone, and is expected to release another beta later this month.