Currently, it's up to the drivers to 'Accept' carpool passengers however there is something you can do to request a ride.
1. Go to the 'Trips' section and select which trip you want a driver for.
2. You should see a list of matching drivers, they will be on a similar route to yours.
3. Feel free to check their profile, which includes a star rating from previous carpoolers, Facebook profile verification and a photo. If you're happy with what you see, you can start a chat with them with something nice like, 'Please can I ride with you tomorrow?'
Hopefully the driver will be happy with the detour (if there is one) and accept you as a passenger.