You should be able to put in {php} {/php} tags on the first entrance to your order stream, something like $_SESSION['group_promocode'] = $_GET['promocode']; And then just put the promocode into the text box when it comes to that point (using other {php} tags to put it into SMARTY (i.e. inside an if statement: $this->assign('hasPromocode', true); $this->assign('promoCode', $_SESSION['group_promocode']))).
I don't like the idea of doing this because it puts code into the View layer. In all honesty I can't stand the smarty template engine because it has {if} and {foreach} blocks (Model / Controller and View should never be together in my opinion). Then again, WHMCS doesn't seam to be exactly MVC, but that's a discussion for another day.
Anywho, yeah. It could be done.. theoretically, not elegantly, and hackishly.. but it could be done.
Cheers,