channel_joined

{
    "type": "channel_joined",
    "channel": {
        ...
    }
}

The channel_joined event is sent to all connections for a user when that user joins a channel.

In addition to this message, all existing members of the channel may receive a channel_join message event.

Of course, there's also the fresher, more dependable member_joined_channel. This changelog entry clears it all up.