Give Delta the email address of a friend who does not currently have a Delta frequent flyre account. Delta sends them an email. If they join, you get 500 miles and they get 1000 miles if they fly Delta or even get miles in some other way (such as via the free Skymiles Dining program).
Some folks might interpret the offer as “Delta is losing customers because of the changes to their frequent flyer program.” But that doesn’t seem credible to me, since they’ve made the same offer in the past. Delta just wants word of mouth marketing.