I'm trying to make a map that will only teleport you when everyone on the server has a ready score of one. I want it to be flexible and allow different numbers of players, but the comparator will output the number of players with a score of one.
I did something like recently, where I wanted to trigger something once all players were in an area. I think the idea will work for you too. Unfortunately I don't think you can do it with a single command block but the circuit is pretty simple.
The circuit compares the total number of players with the number of players matching a condition and outputs a signal once all players meet the condition. I don't think it will work right if you've got more than 15 players due to the max signal strength of redstone.
Let me explain the parts:
A - an input which turns on the detector (I have a tendency to turn off redstone clocks when they're not needed to reduce lag)
B - A simple comparator clock.
C - A command block with the command "testfor @a". You can see it's outputting because it's detecting me. It's actually outputting a signal of strength 1 because I'm the only player in the world.
D - A command block that detects all players with a condition. In my case, I had a "testfor @a[x=...,y=...,z=...]". In your case you'll want something like "testfor @a[score_FOO_min=1,score_FOO=1]" to test for players that have a scoreboard FOO equal to 1
E - The output, which may need to be amplified with a repeater if it needs to travel more than 1 block.