I have seen alot of posting regarding this topic; and perhaps I just don't have the skills to make it happen so I can't how obvious this is, but I need to share the users between WP and CPG. As both products are mature and stable I will assume there is a plugin or reasonable way to accomplish the sharing of users.
There is no official bridge. The official ones are listed in the bridging section of the docs. User-contributed ones might be available, see http://www.google.com/search?q=coppermine+wordpress+bridge