Under Actions in Combat: Ready:
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
Forgoing action and movement on their turns, the group could use their reactions to move up to their speed, towards and off the cliff on the same turn as the sorcerer, allowing the player to cast feather fall, targeting everyone.