Here’s the basic rule for Surprise (Player’s Basic Rules, p. 69):
If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the counter.
I’m not sure how to interpret that last sentence.
For example, suppose that the players try to sneak up on on some monsters, so they roll Dexterity (Stealth) checks. The thief and the champion both roll 14, but the evoker only rolls 9. The DM compares this to the monsters’ passive Perception: a cyclops with 8, a dire wolf with 13, and a giant owl with 15.
The cyclops doesn’t notice any threat, so it’s surprised. The owl notices all the threats, so it’s not surprised. But what about the dire wolf? It notices the evoker but not the rogue or the champion. Which is correct:
- The dire wolf noticed a threat (the evoker) so it’s not surprised.
- It didn’t notice a threat (the rogue and champion) so it’s surprised.
If the former is true, could the rogue and champion take advantage of the group check rule to help the less-stealthy evoker hide from the dire wolf? Group Checks (Player’s Basic Rules, p. 59):
To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds. Otherwise, the group fails.
Group checks don’t come up very often, and they’re most useful when all the characters succeed or fail as a group.
An earlier question covers a couple of related situations, where a rogue sneaks up separately from the rest of the players, or where all of the monsters use a single group Dexterity (Stealth) roll, but not this case where all of the players try to sneak with different rolls.