Take a multimeter and check for voltage on the red lead at the starter when one of your buddies pushes the starter button. If there is nothing then it if your solenoid which is behind the side panel by your left knee when you are riding. Or it could be a faulty thumb switch that you could check if you pull the wires and touch them toghther. If there is power at the starter it is probably the starter. I´m not a mechanic but I had to sort out the wiring on one of these after a buddy of mine royally messed it up. You can go to a dealership for parts but E-bay is definitely way cheaper but you take a risk buying 30 year old parts that you only have pictures of especially electronics. Good luck and post back if you have any other questions or possibly more details as to what is going on.