If your friend gets hurt...
If you can, place crossed skis or a vertical snowboard above just uphill from the injured person to alert others of an injury.
Ask a passing skier/snowboarder to tell the nearest lift operator that you need someone from Ski Patrol to come and help. Be sure the person knows the name and location of the part of the hill you are on.
If there is no one else around to report the injury for you, you will need to ski/ride down and report the injury yourself. After you tell the lift operator, you should go back up to the top of the run, wait for ski patrol, then take them to your injured friend.