define useraccount ( $admin = false ) {
        if $admin  {
                exec { "make_admin": }
        }
}

class users::people {
        @useraccount { "bob": }
}

class users::admin inherits users::people {
        if defined(Useraccount["bob"]) {
                Useraccount["bob"] {
                        admin => true,
                }
        }
}

define admin_users {
        include users::admin
}