Back in 2017, Microsoft restructured the Xbox Insider Program with four separate rings that get updates at different paces. Most of these were only available by invitation, and this later changed so that most of them are accessible to anyone, though certain requirements still exist for the Delta and Beta rings. However, the Alpha and Alpha Skip Ahead rings are still invitation-only, and these are the rings that get new features the earliest.
If you want to be on the bleeding edge of new Xbox system features, Microsoft is opening up the Alpha and Alpha Skip Ahead rings to more Insiders. Starting today, any member of the Xbox Insider Program can fill out a survey called Joining new rings, which is available in the Xbox Insider Hub app. Filling out the survey will let Microsoft know you're interested in joining, but there's no guarantee that you'll actually be accepted, so access is still restricted.
For those unfamiliar, the Alpha Skip Ahead ring is the one that looks furthest into the future. In this ring, you'll be skipping ahead (hence the name) of the next Xbox One system update. Instead, you'll be getting builds from an update after that, which means you'll probably get new features well before they're generally available.
The Alpha ring isn't as forward-looking, and it only gets preview builds for the next publicly available update. However, it's the first ring to get these builds, so you'll get access to those new features before anyone else. Currently, both of these rings are actually testing version 2006 of the Xbox One system software.
Once you've registered your interest in joining these rings, you'll need to wait for Microsoft to tell you whether you can join or not. Those that get accepted should get an e-mail at some point in the future.