If you want to lower admin privileges on individual sites you'll definitely want some kind of role management. The Admin Menu editor just hides/adjusts the menu placement. If they know the full admin link, they'd still be able to access those areas.

What you're looking for is a "membership" plugin. I know that Justin Tadlock had worked on a plugin for wordpress called members and here's a link. It may accomplish what you want, but I've never tested it myself on multi-site. There are several out there if you do a google search.

Also, wpmu premium dev just released a pretty full-featured plugin that I believe would do what you're looking for, but it may be overkill.