I typically make a point to split push and punish anyone who tries to stop me on their own if team fights arn't going well.
In team fights, I look to bait out some pokes and try to see if I can't wear down their mana. Once they seem close enough together and some key CC on their team is gone, I engage on that and do my best to lock down even just 2, but hopefully at least 3, people for a second or two. You will wan't to pop your ult, since because the range on some of your CC has increased range, you can affect more of them that way. Riven has the funky job of disrupting the enemy team like a tank while being easily blown up if the timing is bad, so baiting out CC at the start is critical.
However, if your team has a better engage than you, LET THEM DO IT. Far too many times I see Rivens thinking their hot shit and engaging and immediately dying when they could have let someone much more qualified do it. You almost don't need to bait for CC this way. If this option is used, I suggest bringing as little attention to yourself and just walk in quietly rather than try burning up your mobility needlessly, since her mobility is also her damage.
No matter how you do it, your focus is to CC and put damage on as many as you can. If you think you won't last much longer, be sure to throw your ult out anyway at the place you figure the most people will be at that moment, even if you have not finished your combo. Best case, it saves you, worst case, you put on just a little bit more damage and hope your team cleans up.