[CentOS] user management solution needed

Hey folks,

What is the best way to manage users across multiple CentOS boxes?

Ideally what I'd like to be able to do is have central control over
who has access to which box from a minute-to-minute basis.   e.g. User
X needs access to Box A for 30 minutes - clickity, clickity and they
have access for that long after which their access is automatically
turned off.

thanks,
-Alan

-- 
“Don't eat anything you've ever seen advertised on TV”
         - Michael Pollan, author of "In Defense of Food"