Evolution Studios released some new multiplayer details. It'll be possible to play with at least twelve people online. The studio is trying to get sixteen players working at the moment, but twelve is for sure. With those at least eleven other people, you can voice chat with a usb or bluetooth headset.
There is no splitscreen mutliplayer at the moment, but it may be available for download if the folks want to. There is more to download, you can upload and download replays, so you can watch other's painfull crashes or let people see your most hummiliating crashes!