Of course this spell will work on the entire group. This is exactly what this spell is for.
It says so in the description: up to five creatures. To deny them that is to violate the intent of this spell.
Sure, by a strict turn-based board/computer game interpretation, everybody who jumps off the cliff would plummet to their death untilbefore it's the next player's turn, but that's a stupid interpretation of the rules that violates the intent of this spell. The entire reason RPGs have a GM, is to have someone who can say: yes, this works. Of course it works. It's meant to work.
Focusing on the turn-based abstraction of combat and letting that override roleplay opportunities means you're turning the game into a boardgameboard game. It's not. This should work, because it's what the spell is for. Feather Fall would be a pointless spell if it didn't work.
If you really insist on sticking to the turn-based straight-jacket of the combat rules, I suppose you could let everybody delay their action or ready an action until everybody is ready to jump, but in my opinion, even that is missing the point of the game. The turn-based mechanics of combat isn't what the game is about; they're an abstraction to help you manage combat as part of a larger overarching story. Don't let the game mechanical abstractions overrule the roleplaying opportunities. It's not chess, it's a roleplaying game. You can do whatever you can imagine, not merely what's specified by the rules.