Restricting email flow within a group within an organization

We have a client that has an in house exchange 2010 server.

The customer requests a small group of users to have the ability to email one another within said group without being able to email anyone else within the organization nor outside of the organization. Nor will they be able to receive email from outside of the small group within the organization, nor be able to receive email from the outside.

I have been able to restrict mail flow to within the organization, but not quite sure how to select the specified users from within the organization.

Any tips and advice on this would be helpful. An odd request indeed, but it is for students within a private school.