You've exactly described the use case for Bridging, particularly when the people tracking down answers are doing so by engaging with people who may not be active participants in your customer community. This is a common enough case that we've actually made a bridged, team-facing community part of our new Social Customer Service solution. It may be worth discussing with your rep.
An employees-only place in your customer community can also work well, particularly when the people likely to have the answers are actively engaged in that community and it's more about avoiding duplication of effort and enabling some "behind-closed-doors" discussion about the best way to respond.
The Engage Partners discussion that John referenced is more focused on working closely and in private with specific customers and partners, for example where you have an employees-only community you'd like to allow non-employees into limited parts of for closer collaboration with employees. This can be flipped around as well, for creating lots of segregated experiences within a customer community, but doesn't quite sound like what you're looking for.
Hope this helps!
Thanks Olivia Teich. I had checked out the files/PPTs/etc on the Engage Partners group and didn't see the scenario I was trying to address, so thanks for confirming I hadn't missed it. I'll check out the Social Customer Service solution as well. For now, I'll start working on the employee only space and see how that goes (I'll do it after 5.0.2 goes into Production for us).