Your code should work fine if you removed the form from form.grouped_collection_select
OR
If you really need it and the form is already referring to a user form builder then you can skip the first argument of :user passed to the grouped_collection_from method
Using any form builder to initiate a helper method like (select,collection, grouped_collection, ...etc) will automatically passes its object
to the helper and the helper will expect arguments starting after the object
parameter.
Check this SO question: collection_select method gives error in Rails 3.1.1