Give a user rights to a specific node in Drupal

I'd like the user "student" to be assigned to a content type "Projectgroup". I can do this by adding a user reference to the Projectgroup content type form.

Example:

Projectgroup = Beta testers

Students (user referenced) = Kim, Joel, John.

When Kim logs in, she should only be able to post as "Beta testers".

How can i make sure when the user logs in he/she can only post with the correct projectgroup rights?

You want to assign roles to these students and then control what permissions are available to that role.

http://drupal.org/handbook/modules/user