When PCs want to help other PCs do something, we have to be careful about what we allow because it's easy to break the game by ignoring the probabilities involved. The core mechanics stipulate that when a PC helps another, we're conferring a +2 or advantage to the roll depending on context, but when you allow that and why is important to reason out.
Look to the Fiction First
Of course, group action also has to make sense fictionally. If the PCs are acting in tandem (presumably on the same turn, if the game mode has discrete action), it can be argued that their efforts are coordinated and you should only call for a single roll. But if that's not possible—multiple PCs are trying to sneak and are physically separated in the scene—then they should make individual checks and establish individual stealth scores.
On the odd scenario where they've initially planned together, but then decide to act separately, each PC should individually receive a bonus to what they're doing because of their previous coordination.
When Does the Help, Help?
To determine if the helper can actually improve the situation, ask yourself the following questions:
- Are there already too many cooks in the kitchen? Sometimes a task isn't helped by adding another pair of hands.
- Is the helper more experienced than the PC? If the helper has a higher attribute score or is skilled in the task when the PC is not, then the helper can improve the roll.
- Is it a matter of brute force? The more PCs helping the better when it comes to lifting a heavy tree trunk, but not so much when trying to pick a lock.
You can only ever improve a roll with advantage or a +2, so whether we've got five PCs assisting in the occult research or two, at the end of the day the numbers only support improving the roll by a +2, or with advantage.
You should penalize a group action roll in a countable way, if possible. Remember that the PCs have lots of benefits at their disposal to make something work if they really want it to (fate points, story tags). Therefore, if the point person is trying to help their party of 5 sneak across the battlefield, it's OK to impose a -4 penalty to the check.
If they have time to prepare or the necessary resources, the party can take some effort to mitigate the malus. This would trigger an appropriate check to see if the malus can be negated (typically a success check).
Rolling to Success
Lastly, don't let multiple players roll for the same thing at the same time! This will only destroy the odds of the roll being meaningful. Instead, always force the PCs to choose a point person who leads the effort. If they want to roll again, the conditions need to dramatically change to allow it.