DataGridView BindingDataSource with a Foreign Key

Question

I have a DataGridView that is bound to a BindingList<T> of a custom business object, with fields DeckID, Name, UserID, Size, and Notes.

This is being called up from an MSSQL database via an IDataReader object. Now, my problem is that UserID is an integer that, in my database, is a foreign key to a table called SystemUser, with a Username field.
What I want to do is grab the Username from the SystemUser table in my database, and display that, based on whatever UserID, display that name. This was all very easy in WebForms, using a GridView, but not so much on a WindowsForms app.