I don't understand.

WHY are you offering 100% discounts?
WHY would you want your customers to be automatically added to a 100% discount group? You may as well set all your products to "free".

1. Your customer signs up.
2. The customer doesn't get placed into a group-discount group until the administrator moves them.
3. Once they are in the discount group, their discounted prices will be shown.
This has nothing to do with downloads.

Downloads are always available immediately after purchase if payment has been completed properly (ie: the order status is set to "Processing" or whatever other setting you've configured your site for)