I actually think my favourite implementation of group action is in Blades in the Dark (if you have it, read "Lead a Group Action" on page 134). In a nutshell, here's how it works:
When everyone takes the same action, one person is identified as the leader for the action (usually the person with the highest rank). Everyone involved in the action then rolls. The group counts the single best result as the overall effort for everyone who rolled. However, the character leading the group action takes 1 stress for each PC that
failed the roll. Basically, the leader is covering for the less experienced in the group.
This is ripe for interpretation in Forbidden Lands / Year Zero. For example, you could rule that failures count against the best success instead of adding stress. Here's an example.
I like this better than taking Stress, because if the leader only gets a single success and everyone else in the group fails, it shouldn't make sense for the leader to somehow cause the team to succeed regardless. They were all crap and (s)he barely did a good job.
Saying that, taking Stress could still be an option, ruled thus: "You either reduce the number of successes, or take Stress equal to the number of failures."
PC1 (leader): rolls Sneak (7d6) and gets 3 successes
PC2: rolls Sneak (5d6) and gets 1 success (this result is ignored)
PC3: rolls Sneak (4d6) and fails (one failure)
PC4: rolls Sneak (3d6) and fails (one failure)
Result: The 3 successes from the best roll are reduced from 2 failures, giving one success.
Naturally the group of guards would also roll Scout
in exactly the same way. You take the best result and (since it's passive) only give the option to reduce the successes by the number of failures, rather than having the lead guard take stress.