code: public synchronized void addUserToRole(String userName, String roleName)
      {
          HashSet userSet = (HashSet)_roles.get(roleName);
          if (userSet==null)
          {
              userSet=new HashSet(11);
              _roles.put(roleName,userSet);
          }
          userSet.add(userName);
      }
docs: Add a user to a role.
  @param userName
  @param roleName