Profiles site with access levels

This sounds like it’s somewhat similar to a project I recently put together. You should check out the Role Scoper plugin: it will be an important puzzle-piece in making this work. It’s a confusing plugin, but it’s the way to go.

“Watchers” sound the same as a default role, “Subscribers,” so you could stick with that. Alumni could be a custom role based on Subscribers with elevated privileges as follows:

You create either two custom posts types, or two categories, or two terms in a custom taxonomy, whatever: Public and Private. Alumni can post to either one, and can read either one. Subscribers (or Watchers, if you prefer) can read only Public.

If you want to set up custom taxonomies or post types, you can do it by hand, and the methods are well-documented, but there are plugins that make it easier. I used the Types plugin.