IFulltextQuery<TDocument> Having(
	Expression<Func<TDocument, bool>> predicate
)

Function Having ( 
	predicate As Expression(Of Func(Of TDocument, Boolean))
) As IFulltextQuery(Of TDocument)

using (IFulltextSession fulltextSession = fulltextStore.StartSession())
{
    var results = fulltextSession.Query<Product>().
                                  Match("a product").
                                  GroupBy(p => p.VendorId).
                                  Select(p => new
                                  {
                                      p,
                                      Count = Projection.Count()        
                                  }).
                                  Having(p => p.Count > 1).
                                  ToList();
}