Refer-a-friend / Gifting solutions

hey everyone,

I would like to add refer-a-friend and gifting options for users on my side project. I am looking for a list of free options that i) allows users to share a sign up link to their friends/networks and gives them a credit/discount on their next month's subscription, and ii) allows users to 'gift' another a credit toward next months' subscription(s).

thanks!