{
"type": "group_left",
"channel": "G02ELGNBH"
}
The group_left event is sent to all connections for a user when that user
leaves a private channel. Clients should respond to this message by closing the
private channel - this means that when a private channel is left from client A, it will automatically be closed in client B.
The channel value is the string identifier for the private channel.
In addition to this message, all existing members of the group will receive a
group_leave message event.