RTR, that won't work at all for this scenario as groups only exist in a course. The OP is asking for different courses/categories to be visible to certain people. It is not possible in the standard Moodle.
Quintin, I know you've said you can't use Plugins or change code. Have a look at http://www.iomad.org/ though, it's a fork if Moodle that will do exactly what you require.
