Joins the session by providing a valid session id. Accept header means that the user accepts the flag warning before joining the host.
{"msg":"join_server","instance_guid":"my_session_id_here","passkey":"Insert passkey here.","invite_code":"uuid_generated_code_here","accept":false or true}

The server will check if the session is not full and if it isn't then you will receive a connected message:
{"MSG":"connected_session","SocketID":1,"InstanceOwnerID":0}

Your presence will be announce to the other clients connected:
{"MSG":"joined_session","SocketID":1}

Then you will receive other's presence:
{"MSG":"joined_session","SocketID":0}
{"MSG":"joined_session","SocketID":2}

The Instance Host will also receive your presence in the session:
{"MSG":"joined_session","SocketID":1,"IPAddress":"123.123.123.123","Flags":<An array of flags shown here or nulled if the flags aren't found.>}

If you have been banned from the session:
{"MSG":"banned_from_session","Reason":"You broke rule #1","ExpiresOn":<epoch time converted>}

If the session has whitelist enabled and you are not on the whitelist:
{"MSG":"not_on_whitelist"}

If the session is full and cannot accept anymore till one slot is available:
{"MSG":"session_full"}

If the session is locked:
{"MSG":"session_locked"}

If the session's password is incorrect:
{"MSG":"invalid_password"}

If you have already been connected to the session before then the server will respond:
{"MSG":"already_joined"}

If the session is not found:
{"MSG":"session_not_found"}