Display user registration date

get_current_user_id() give you the user id of the logged in user. And that is: you.

You have to get all users:

<?php
        $users = get_users();

        foreach( $users as $user ) {

            $udata = get_userdata( $user->ID );

            $registered = $udata->user_registered;

            printf( '%s member since %s<br>', $udata->data->display_name, date( "M Y", strtotime( $registered ) ) );
        }