Turn it on
Add the 'Refer a friend' action to your campaign and set what a referral is worth. That point value is what each successful referral awards — there's no separate setting.
Once someone has entered, their personal referral link appears on the entry page with a copy button.
How crediting works
A referral is credited when someone arrives via a ?ref= link and enters for the first time. Each referral is its own record, so ten referrals award ten times the points.
Self-referral is rejected, and an entrant who was already attributed to someone else can't be re-attributed.
Watching it
The campaign analytics page has a top-referrers table. If one code is running away with the campaign, that's usually worth a look before you draw.