You can try adding this advanced script to a Read ACL for sc_req_item.work_notes. 

Type: record
Operation: read
Name: sc_req_item.work_notes
Role: sn_request_read

var answer = false;
var userObj = gs.getUser();
if (userObj.isMemberOf(current.assignment_group.sys_id))

{
answer = true;
}
else
{
answer = false;
}