If you're in the loop with a group of friends you know what's going on and get updates on a given situation.
A person who is in the loop is aware of what's happening in a specific context, and may have some special inside knowledge that other people lack: "I'm in the loop on the surprise party, so you don't have to explain anything." The opposite of in the loop is out of the loop, used to describe someone who doesn't have, or isn't being given access to, information.