Temporary usergroup promotion?

[Snakehill]

My board has a store in which registered members get discounts on all products (the same page is used for non-registered members but these just won't see the discount links and prices). The only problem is that most people signed up just for the discounts and not to become part of the community.

I would like to set it so that whoever wants a discount, should have posted at least 5 posts within the last 30 days, so that only more active members get the discounts. I guess best would be if people would be added to an additional usergroup for the time being, but how to set this up?

Doing this manually would be no option, due to obvious reasons.

Thanks!