A ping means they have found you, however a light ping means they do not have a good fix and you can still evade, heavy ping "dive dive" hahaha.
They definitely work as a group. Generally one will always be listening with passive will the others ping. One part about the game that is not very realistic is that the ones listening can still hear you even when DC are exploding so you are not safe even when there is a ton of noice in the water.
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