I think the invitation system is tied in with the UCD and LCD car rotation. Next to last time the Cobra Daytona Coupe showed up I got three Ferrari invites. I bought it and a bunch of Ferraris only to miss out on the McLaren F1 road car and 3 more stupidly expensive Hagerty rides. This time around the Cobra Daytona Coupe showed up and I got a Lamborghini invite. Also the 911 Strassenversion started changing colors, again trying to entice me to buy several of them.
Four 20 million credits cars are about to drop in the LCD almost on top of one another. If players spend all their credits on other things, they'll have to stick with the daily grind for another 3-4 months to get the expensive cars or they'll have to spend hundreds of real world dollars on MTX's to get them.
Watch the boards and wait for the 917k or the Cobra Daytona Coupe to show up in the LCD. I'd almost bet that's when the invites start dropping. In fact, if they don't, I have a 1998 Ford Lightning in game that you can just have.