Restricting registration and access
 

We are developing a forum that will be used by 3 different organizations. Each organization has a unique email extension. We will be making the forum private by only allowing those to register who have an approved email extension. All the users will be posting messages to the same forum. We want the user only to have access to those posts that have been generated by someone in their organization (same email extension). Eventually we want to incorporate the functionality that would allow a user to select which of the 3 organizations he can view and interact with through posts. What would be the best way to do this? Is groups the way to go - if so how?