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group_leave message event

A member left a group

Compatibility: RTM

    "type": "message",
    "subtype": "group_leave",
    "ts": "1358877455.000010",
    "user": "U2147483828",
    "text": "<@U2147483828|cal> has left the group"

A group_leave message is sent when a member of a private group leaves that group.