Grand Theft Auto Online, the sprawling open world online component of GTA5, is all about hanging out with your friends and having a good time. It's so open, though, that your more unscrupulous buddies can take advantage of you, so Rockstar has put a few pieces in place to prevent that very thing.
In a hands-on, IGN notes that during a heist with a buddy, the passenger gets to decide the split of the take. If he tries to rip you off, though, he runs the distinct risk of getting shot and ending up with nothing.
As you might expect from a game named "grand theft auto," other players will be able to steal your cars, giving you a true taste of how all those random NPCs feel when you do it to them. Fortunately, though, you can mitigate this threat by insuring your cars, which presumably means they'll be automatically replaced if anything happens to them.
You'll also be at constant risk of losing whatever cash you have on you at the time. If you die, your wallet will just sit there with your body, available for any of your friends to snatch up. For that reason, it's important to make regular visits to the ATM and deposit any money you've recently scored from heists. You would think the bank would start to get suspicious.