How to implement a limited feature rollout (language agnostic) to your users?

I would like to know some common or best practices of rolling out a new website feature to a select group of the userbase.

The users could be, for example, based solely on a percentage of your overall user base (10%). The rollout should be customizable (configurable) and support any number of features. It would also be useful to associate rollouts to specific user roles or privileges (ACL).

So, in essence, what is an architecture that would scale reasonably well?

As for the language agnostic portion, you can either provide pseudo-code, general concepts or ideas, or snippets from your preferred language to get your point across.

Links to any examples or tutorials are welcome.