Google OAuth2.0 Scopes page: https://developers.google.com/identity/protocols/oauth2/scopes

To access employeeId, the required OAuth2.0 scope for read only purposes is: https://www.googleapis.com/auth/admin.directory.user.readonly

For full read/write, use: https://www.googleapis.com/auth/admin.directory.user

Note that you will need to enable/add the Admin SDK API. The employeeId is part of the externalIds array in the user resource. Retrieved by calling:

GET https://admin.googleapis.com/admin/directory/v1/users/{userKey}

Response:

"externalIds": [
  {
    "type": "custom",
    "customType": "employee",
    "value": "12345"
  }
]