Hi,
It depends what code you want the user enter.
HikaSerial have a consumption plugin called "group consumer" ; it allows to affect a user group to a logged user when he used a specific serial. But once the serial will be used, he couldn't be used again.
Otherwise, you can see to use a user custom field which will be display during the registration and then, using a little custom plugin, you can change the user group after the user creation (depending the content of the custom field).
So, it would be more something like a pass-code to unlock the user group.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.