How much power do power stations in parallel supply?
It's quite simple. Each synchronous machine can act as generator or a motor. What makes the difference is the angle of rotor with respect to the rotating magnetic field. If the rotor has a leading angle, then it's a generator mode. If it's lagging then it's in motor mode.

The regulating loop just adds some negative torque to the generator, so it increases the generated power and keeps to the nominal output power/curent. For large distribution network this is also driven by a schedule, a predicted demand. 

EDIT:

Before you connect the generator with the grid you have to sync it. The output has to have the same frequency, phase shift and amplitude. Then you do connect on mains. Now the generator will run at exact speed as mains frequency. If you lower the throttle the genset will become a motor until overloaded, if you apply throttle it will turn into a generator as long it is not overloaded.