{
"type": "mpim_joined",
"channel": {
"is_mpim":true,
'is_open':false,
…
}
}
The mpim_joined event is sent to all connections for a user when that user
joins a multiparty DM. In addition to this message, group_joined and
group_join events will also be sent for backwards compatibility, since MPIMs
can also be treated as groups. Clients that are MPIM aware can ignore all
group_join and group_joined events when the channel's is_mpim flag is
set to true. MPIMs will always start closed for everyone but the creator.