Apparently, the test build will initially only run on the OnePlus 2, so owning one of these is probably a prerogative for participating. Other requirements include being willing to flash updates twice a week, being online for at least 30 hours a week, an ability to submit bug reports and timely feedback, with past beta-testing experience being a serious plus, of course.
OnePlus' handsets, except the One, run on a homemade user interface, named either Hydrogen OS or Oxygen OS. The former is meant for the Chinese market and comes without Google framework, whereas the latter is installed on the internationally-sold handsets, and comes with all the Google apps you're used to seeing on an Android smartphone. That aside, both are pretty similar, as they are just variants of the same system. It has a near stock Android feel, due to the fact that it doesn't re-skin or modify features extensively. Hydrogen / Oxygen OS, much like Cyanogen OS, adds a lot of customization and poweruser options, but tries to not get in the way of a vanilla Android experience.
The OnePlus One runs on Cyanogen OS — the phone was produced and launched long before the two companies' falling out, and it has been receiving numerous software updates since. We are left under the impression that the One will get a Cyanogen Android 6 Marshmallow build, while an official Oxygen OS ROM will be available for users who prefer to flash that instead — just like the Lollipop Oxygen ROM, which is currently available for the OnePlus One. The Weibo post which announces the beta test says something along the lines of (translation isn't perfect) “OnePlus One and OnePlus X users, be patient — you will really enjoy Marshmallow Hydrogen OS”, further confirming that the two smartphones will be updated.
So, does this mean that the OnePlus One will be getting Hydrogen? Only time will tell and, at this point, it seems it's going to be a couple of months before we see Marshmallow on the OnePlus handsets.