You might peruse (if you haven't) Category:User rights extensions for what you need. In my experience, none of them will give you exactly what you want without risking trouble down the road. There is a trend of un-maintained extensions in this flavor, perhaps due to how tricky it is to accomplish in the first place.
We currently use Extension:Lockdown to protect content that is considered secret (and there's not much of it). It's based on defining namespaces and associating each with groups allowed to see them. It's not perfect, but has a history of being maintained and gets the job done. We used Extension:SimpleSecurity for a couple years, but it went unstable after some upgrades. We cut it loose when newer features of MW were too useful to resist. You should check out Extension:Access Control List (a spinoff of HaloACL from SMW+ ). If it gets/maintains a strong following I'll probably be checking it out soon. It could be the tipping point for corporate mediawiki adoption.
Further, I'd strongly recommend examining what reasons you have for limiting access to users across the same mediawiki installation. Mediawiki works because it connects a community who all have the same interest/stake in the content and are empowers them to improve it. If the users are deprived this ability, you are left with a CMS that tracks history. If you really want to reliably separate who can see what, install a wiki for each group.