When the entire group makes an ability check together, then the results are taken all together. The group either succeeds or fails the ability check as a group, not as individuals.
If at least half the group fails the check, then the entire group fails the check. And vice-versa.
These are probably most commonly used for Perception and Survival checks – can the group notice important details of a room, or can they find their way out of the forest.
DM – Tip: If one person rolled a Natural 20, and the others rolled terribly, have the others roll Insight to see if they believe the player with the Nat 20. They might not automatically believe which way is out!