How to Put $expand Property in groupby query in ODATA

I am using WebApi 2 with odata 5.3.1.

I am using groupby query, by implementing my own GroupBy() custom function.

http://localhost:51738/odata/Document?$apply=groupby((Category),%20aggregate(Documents/$count%20as%20Total))

But in new database the Category Property is a navigation property & we can achieve it by using $expand. So how to use $expand query inside groupby query?