Opportunity Work Group functionality design question

Any design ideas?

Requirement: Opportunities have Work Groups.  Provide a mechanism where the list of members of a work group are associated to an Opportunity. 

Known items:

1. Work groups consist of users of CRM and non-users of CRM.  Work groups range in size from 10 to 80 people.
2. Non-users of CRM are all employees and are defined to Active Directory.  
3. Purpose of non-users of CRM are to provide a list of people who are on the opportunity team and their role (standard list of opportunity roles)
4. Active Directory is integrated with our CRM
5. Users of CRM at a minimum will have read to an opportunity. 

What are some ways that you would implement this functionality?

What pitfalls would we have if we used the contact entity and used that to populate the Work Group?  Should we go custom in other ways?

Diane