Dear Friends,
We are glad to announce the release of our new free add-on “Auto Add User To Group on Register”!
Extension will automatically assign new users to the selected group on register.
Compatible with versions 4.0.x / 4.1.x / 4.2.x / 4.3.x / 4.4.x / 4.5.x / 4.5.x / 4.7.x
Demo: here
Download: here
Please feel free to ask any questions!
Best regards,
—
Valentin
part of hungryweb.net
You could expand this by adding the options "Add to group if “discount coupon” used. That way you could build a list of customer who have already showed they are buyers if even the right enticement 
. Or you could give coupons to a group and filter them into a group on your site. For example I could give a coupon to my Facebook friends and then tag them as buyers if they use the coupon.
Compatibility: 4.3.x / 4.4.x / 4.5.x / 4.6.x / 4.7.x
[+] Easy support
More details : hungryweb.net/…/auto-add-user-to-group-on-register.html
THANK YOU !