Invite 3 people with your link, which is on the Subscription page. Each one counts once they've created an account and logged their first dive. They need to open your link before signing up, on the device they sign up with.
When the third one logs a dive, we'll let you know and you can start your free month whenever you like. It lasts one month, doesn't renew and includes everything in PRO, with its own 30 photo and 100 description identifications. You can earn it once per account.
Your link and your free month only appear while you're on the free plan. If you've already earned the month, it waits until your paid plan ends. If you subscribe while it's running, your subscription takes over and the rest of the month is lost.