I am working on workspace-local group APIs on Azure Databricks though what @Retired_mod suggested did not work to me too. I checked the payload sent from Databricks workspace frontend and the following worked to me. Has API signature been changed but document hasn't been updated?

{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:PatchOp"
  ],
  "Operations": [
    {
      "op": "add",
      "value": {
        "members": [
          {
            "value": "0000000000000000 (replace this with your Databricks USER ID)"
          }
        ]
      }
    }
  ]
}

Edit:

I am using PATCH `/api/2.0/preview/scim/v2/Groups/{id}` API. I found another doc which looks like v2.1 API reference and the signature looks quite similar to what I used.

Account SCIM 2.1 Documentation (databricks.com)