From the responses it seems that the invite is linked to having Southwest credit cards and/or the promotional companion pass. Indeed, I carry a Southwest credit card for the free (or refunded) wifi on the plane. Now that every RR member is getting free wifi on the plane in a week or so, maybe this gesture is to keep people from terminating their credit card after the wifi perk disappears? If so, it would have been nice if Southwest had given us an explanation of why us.