It is easy to confuse encryption keys and public/private keys. One is like a door knob, you need a key to both lock and unlock it. While public/private keys are like a padlock, the public key can close the lock but only the private key can open it.
This becomes useful when we want to have a private conversation. Alice and Bob exchange padlocks. Now Alice can write a message that only Bob can open. When Bob replies to Alice, he uses her padlock so only she can open his message.
Simple enough, but what if we want to have a group conversation? Alice creates 4 copies of an encryption key, which will let everybody in the group both lock and unlock. She now can invite Bob, Carl, and Dave to the group by sending them the key by using their individual padlocks.
Once everybody has the same shared key, they can now write and lock a message to the group that only the group can see. Allowing everybody to pass messages around.