A simple Slack Workflow that adds member who leaves again.

no_body_leaves.slack-workflow.json

{
  "workflow": {
    "title": "No Body Leaves",
    "description": "Invite who ever leaves the channel again",
    "icon": "",
    "input_parameters": {
      "Ft08U92KCRPH__user_id": {
        "type": "slack#/types/user_id",
        "name": "Ft08U92KCRPH__user_id",
        "description": "The user who left the channel",
        "title": "The user who left the channel"
      },
      "Ft08U92KCRPH__channel_id": {
        "type": "slack#/types/channel_id",
        "name": "Ft08U92KCRPH__channel_id",
        "description": "The channel that the user left",
        "title": "The channel that the user left"
      }
    },
    "steps": [
      {
        "id": "2bd12470-d4a8-4e4d-b38f-ecccddb7a912",
        "function_id": "Fn0107",
        "inputs": {
          "user_ids": {
            "hidden": false,
            "locked": false,
            "value": [
              "{{inputs.Ft08U92KCRPH__user_id}}"
            ]
          },
          "channel_ids": {
            "hidden": false,
            "locked": false,
            "value": [
              "{{inputs.Ft08U92KCRPH__channel_id}}"
            ]
          }
        }
      }
    ]
  },
  "triggers": [
    {
      "name": "Someone Leaves",
      "type": "event",
      "inputs": {
        "Ft08U92KCRPH__user_id": {
          "value": "{{data.user_id}}",
          "locked": false,
          "hidden": false
        },
        "Ft08U92KCRPH__channel_id": {
          "value": "{{data.channel_id}}",
          "locked": false,
          "hidden": false
        }
      },
      "event": {
        "event_type": "slack#/events/user_left_channel",
        "channel_ids": [
          "${{ ADD SLACK CHANNEL IDS HERE }}"
        ]
      }
    }
  ]
}