I can get the custom sections (Parameter Group) on a custom form (Category) with:
GET /attask/api/v15.0/ctgy/{customFormId}?fields=categoryParameters:parameterGroup
This gives me the Parameter Group ID, but I don't see any sharing/permission fields in the Parameter Group object fields list when I check the API Explorer.
The goals is to update the sharing permissions of all instances of a particular custom section across workfront to be admin only.