Haven't recieved my Murcielago yet, the Shelby GT500's were sent out yesterday from the bounty hunter though.
Rivals mode covers monthly events which T10 set up, and a few other permanent ones such as Autocross, Drift and some others, in rivals mode there is usually a set criteria that your car must match to make it different to the normal leaderboards, that and you get money for beating people. You can set times in rivals mode and then maybe message your friend and they could try to beat your time, I'm not sure if there is a button that sends the challenge to your friend though or if you can only do it by message. T10 will also sometimes get some 'celebrities' or community bounty hunters (are these staff or just people from the community?) to set times which if you beat you can win a prize. Sorry for this appalling explanation, if I can call it that, I hope I've made something clearer.