It is possible to send a discount coupon to all customer who register. To do this please follow the below steps:
1) Promote>discount coupon> create a coupon with the required offer ( you can keep the no. of redemptions as 'unlimited') >copy the code of this coupon.
2) Settings>email notifications> registration email> paste this code in the template email stating the offer.
This email is automatically sent to all customers who register on the site and therefore, it would give all registered customers the applicable discount coupon.