Hey, @silly-rain-71204! There isn't any formal documentation yet (I can create a ticket to do so) but in the meantime, I can answer any questions you might have. Currently, it's a way to define a group of users/organizations/team members/etc that need to be referenced often with feature flag targeting. You can define what attribute you want to define them with (ex - userId) and then manually type our a comma-separated list of those userIds.
Then, when creating feature flags, you can build rules based on whether or not a user is/is not in a saved group.
We have discussed adding the ability to define these groups via SQL, but that hasn't been prioritized yet.