This seems to be a difficult solution to search for at the moment. The MongoDB .NET driver documentation links to the unit test files on GitHub for examples on aggregation related items, but the links are broken. Looking at the repos on GitHub is useful to find the actual tests, but for the specific problem I was trying to solve, it wasn't entirely helpful.
Basically what I was trying to do would be relatively easy with SQL -- a SELECT DISTINCT from a table/collection based on multiple fields. I really just wanted to get the unique combinations of several fields at the database layer without having to retrieve all the documents from the collection, and then figuring out which were distinct.