If the requirement to make a field editable only if the logged in user belongs to a group with the with a specific parent group set I think the best way to do this would be with an ACL, which would control whether the field is editable from a form or a list. Outside of that, you could probably use a display business rule that determines whether the user should have access, writes true or false to a scratchpad variable, and use a client script to set the field read only or not. This would only apply to the form and is generally not as secure as an ACL.
