This is what part of the email said---which was very misleading:

"Membership is by invitation only, and you can earn AAdvantage bonus
miles just for participating.

MEMBERSHIP IS FREE, AND YOU GET A VALUABLE WELCOME GIFT!
When you join e-Rewards you will be able to select a Welcome Gift of
1,000 AAdvantage(R) Bonus Miles. Or you can choose $5 in e-Rewards as
your Welcome Gift, putting you well on your way to your first reward of up
to 2,500 AAdvantage bonus miles.....It's yours free as a special
thank-you for joining."