How do I allow the easy association of content types?

As an example of what I'm trying to set up:

The site I'm working on has regional groups - a many to one relationship with each region has many groups but each group only has one associated region.

The list of regions may grow with time, as will the list of groups belonging to each region. I want to make it easy for site administrators using Perch to associate groups with regions as they create and edit them.

My ideal answer would be a drop-down field when the group is set up and edited that populates with a list of currently available regions.

Has anyone come across something similar, and what sort of approach did you take?

Thanks, Alex