You must enable javascript in order to use the Slack API Documentation. You can do this in your browser settings.
Go to Slack

group_join message event

A member joined a group

Compatibility: RTM

    "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.