code: public void setUsers(Set<PrincipalUser> users) {
          SystemAssert.requireArgument(users != null && !users.isEmpty(), "Users associated with a namespace cannot be null or empty.");
          this.users.clear();
          if (users != null && !users.isEmpty()) {
              this.users.addAll(users);
          }
      }
docs: Sets the users associated with the namespace.
  TODO : Review if users can be empty.  What if only the owner should have access?
  
  @param  users  The set of users associated with the namespace.  Cannot be null or empty.