While I don't believe you can have the ldap creation auto-add people to cohorts, you can bulk add people using Bulk User Actions. It all depends on how you set things up and what you want. Once people are assigned to cohorts, you can assign the cohort to as many (or as few) courses as you like. These cohort enrollments can include groups assignment if you want. If you want an additional person added to all the courses a cohort is assigned to, you just add them to the cohort and Voila! they are enrolled.
Cohorts can be at the site level or in category levels. If you only want specific cohorts available for specific course areas, that's how I'd do it. I keep several years (we track training by year) in one site and each year is it's own category...so I create cohorts in the year categories, since people move, change jobs, etc between years. Initially I had site cohorts but found that categories work better for me. It ensures the cohorts are only available for the courses I want them available for.
It also sounded like you wanted to enroll students in course categories. I can't express how much I discourage that. Primarily because category enrollment isn't "real" enrollment and while the student might be able to access the course, they probably wouldn't get a grade and it would be VERY confusing. Also, you have to fiddle with site settings to get it to work because it used to be possible to enroll in categories and no one thought it was a good idea, so in 2.X that "feature" went away.