The auto generate mechanism of RadGridView currently works only with a DataSet object. You must set DataSource to your DataSet object and DataMember to the root Table, in your case DataMember = "Invoices". Your DataSet also must define relations between the inner Tables.
As Tim has suggested, you can find additional information in our online documentation.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.