I don't know if it's every year or every 2 years or whenever they feel like it, but the last 2 AGR "refer-a-friend" offers were (IIRC) during the winter.

If your trip is not before the winter, you may want to wait to join AGR - but still purchase your tickets ASAP! You can sign up and receive credit for trips at any time prior to travel. And in fact retroactively for any trips up to (IIRC) 21 days prior to AGR signup! (But if you sign up before travel, it posts faster.)