Contract of Growth

When Contract of Growth enters the battlefield, you sign this Contract
{g}{g}: Target player signs this Contract
{3}: You sign this Contract. Any player may activate this ability. Activate this ability only as a Sorcery
Whenever a player who signed this Contract casts a creature spell, that player puts X +1/+1 counters on target creature they control, where X is the number of people who have signed this Contract