Use Private Forums
A private forum is a discussion forum that allows individual users to have private conversations with one group of registered users. Individual users cannot view each other's entries.
For example, consider a forum used to review, discuss, and accept construction contracting bids. The discussion-forum manager can create a group called General Contractor, and members of that group can review bid proposals. Each competing company places their proposals in the private forum so that the general contractors can review and discuss them. Competing companies cannot view each other's proposals.
The access controls specified by the creator of the private forum determine who can or cannot view and respond to an entry. When you create an entry, other individual users do not see it (it appears as if Forum "skipped a number" in the entry list).
All other features in the private forum are identical to those of a standard discussion forum.