"mainMenuIcon": "/webapps/3dfindit-enterprise/plugins/plugInDefault.svg",
    "mainMenu": [
      ["customButton1_Group1", "customButton2_Group1", "customButton3_Group1"],
      ["customButton1_Group2", "customButton2_Group2", "customButton3_Group2"]
    ],

Sub-arrays create groups. However, this only works for the "mainMenu" toolbar, as the others normally only display individual buttons.

Button with subbuttons in the main bar, organized into 2 groups separated by a dividing line

In the following example, the buttons are assigned to two different groups:

    "mainMenuIcon": "/webapps/3dfindit-enterprise/plugins/plugInDefault.svg",
    "mainMenu": [
      ["customButton1_Group1", "customButton2_Group1", "customButton3_Group1"],
      ["customButton1_Group2", "customButton2_Group2", "customButton3_Group2"]
    ],

      "customButton1_Group1": {
        "type": "VBS",
        "icon": "/webapps/3dfindit-enterprise/plugins/3dFinditEyeColor.svg",
        "name": {
          "de-DE": "custom button 1 group 1 de",
          "en-GB": "custom button 1 group 1 en"
        },
        "script": "/plugins/vbb/custom1_startup.vbs",
        "tooltip": {
          "de-DE": "custom button 1 group 1 de",
          "en-GB": "custom button 1 group 1 en"
        }
      },
...
      "customButton1_Group2": {
        "type": "VBS",
        "icon": "/webapps/3dfindit-enterprise/plugins/3dFinditEyeColor.svg",
        "name": {
          "de-DE": "custom button 1 group 2 de",
          "en-GB": "custom button 1 group 2 en"
        },
        "script": "/plugins/vbb/custom1_startup.vbs",
        "tooltip": {
          "de-DE": "custom button 1 group 2 de",
          "en-GB": "custom button 1 group 2 en"
        }
      },
...
      },
...