A message was sent to a channel
{
"type": "message",
"subtype": "group_join",
"ts": "1358877458.000011",
"user": "U2147483828",
"text": "<@U2147483828|cal> has joined the group"
}
A group_join message is sent when a member joins a private group.
If the user was invited, the message will include an inviter property
containing the user ID of the inviting user.
When an event occurs, we will send an HTTP POST request to your Request URL. The outer payload is uniform across all Events API methods, and contains fields like team_id and event. Learn more