Default value for registration

Registration on my site is done by a person other than the one being registered. Eleven people all belonging to different clubs are registered as editors for their club.
They are only allowed to register new members for their club. Each registered person has a club_index field that points to the index field in the clubs table.
So the editor logs in and goes to the registration page and enters the registration information for his new member.
Using the standard registration page, with minor modifications to use a menu for the user level, I can get the page to work - but with no club_index.
A new user can only be registered to the club to which the editor belongs, so a session variable supplies the club_index shared by both the editor and the new user.
This club_index does not change, and as far as I can see, it should not need to be validated, as it is provided by the system.
So how do I get this session variable's value transferred into the club_index field of the newly registered user?

On a none SA form, I would usually set the value of that field to a hidden field on the form, and the session variable would be bound to the club_id field in the insert record behaviour.
However, this does not seem to work in SA or I am doing it wrongly, as all the fields on the form change from yellow to green.
When I click the submit button, I get the "Invalid information, please check your entries and try again."
All entries are correct as no other validation errors are shown, yet this error still persists, so it must be a problem with the hidden field. Gets a bit wearing after a while.
I could just insert the user without a club_id and then do an automatic update it on the next page, but that seems to defeat the object.
Just what is the purpose of the hidden field in SA? How can one use it? There is no documentation for this item anywhere that I can find, and it obviously needs some.
All suggestions welcome.