Use wards if you split push so you can see them coming a mile away.
Thing is, your team needs to be always doing something and for a reason. If they're grouping up and trying to push mid, you can either group up yourselves are try to apply pressure on the map.
If the enemy team sends multiple people after you while split-pushing, you just taken pressure off your team. Use wards, recall immediately (when you see them leave the group, not as a reaction when you see them) and regroup with your team. Then overwhelm them with more numbers (since you got rid of a few for their tower, see?).
In a group, positioning is key. Riven, imo, can be considered a bruiser even if built full ad. Either clean up teamfights, or be the second initiator (typically you'll want a tank to lead the charge). People really underestimate this.
Just yesterday I was playing Sejuani and told my team we had to force a fight. The enemy team blew all their skills on me, the frontline, allowing the carries to deal as much damage as possible. Unfortunately, the Katarina got out of position (it wasn't time to clean up yet) and accidentally threw the game. But if she had waited, we would have won the teamfight because the enemy focused the tank due to our superiour positioning.
Just some food for thought.