You can do it by simply making a method in your model.
In your model (Building or Submitter?), add the following:
def full_name
self.first_name + self.last_name
end
then use this method in the grouped collection helper method like:
<%= f.grouped_collection_select :submitter_id, Building.order(:name), :submitters, :name, :id, :full_name, :prompt => "Select name"%>