Invites and referrals
The code that gets a friend in, where to find yours, and what it does and does not earn either of you.
Every account has a six-character invite code, in Settings, then Invite Friends. Sharing it hands over a link; the code on its own still works if someone would rather type it at Enter a code.
How many
A code is good for 10 signups. The page shows how many are left and lists the people who joined on yours.
When it attaches
A referral attaches at signup and only at signup. Entering a code on an account that already exists does nothing, which is deliberate: a referral records who brought someone to Surf, and that is a fact about the moment they joined.
An invite is not a payment. Neither side earns money, a plan or ranking from it, and there is no referral tier to climb. If you want to give someone a paid plan, that is a separate feature: see Gift a membership.