User Profile Lock

Searched the forums and Google but not finding much on my situation.

I have a client that wants 2 versions of their blog, one public and one private. The private blog needs to be accessed by individuals but all using the same login.

Is there a plugin that would allow the admin to lock down the multi-person user account so that someone doesn’t go in and mess with the profile, change the password, or do anything else?

Or am I going about this all wrong from the beginning? Is there a better way to have a private blog yet let users login without having to create a bunch of new accounts? This client has many “students”, probably in excess of 100 and they change on a monthly basis.

Any suggestions would be greatly appreciated!

Thanks