From my personal experience from today, I went to join a party and tries to spawn Cassie,

1) 3 Parties ( killing same morbol )
2) Sharing Chains, we had 3 ~ 4 Chain (30 kills)
3) Cassie Spawned
4) Instance was 130+ people

I assumed Chaining do plays a role

iirc, Yoshida too said that he will tries to adjust the AFK leecher from the base in which it might or might not scale with existing instance players count