Provide ability to limit a forum discussion by invitation either to certain users or groups

Provide ability to limit a forum discussion by invitation either to certain users or groups

We have use case scenarios where we would like to create forum discussions and not have them accessible to everyone in the network.  

Please provide the capability to have different access levels to a forum discussion:  all, group(s), invited individuals, etc...

Having everyone have access to all forums makes this feature unusable for us.