membersince.php

<?php
namespace ShowMemberSince;

add_action( 'plugins_loaded', 'ShowMemberSince\init' );

/**
 * Adding needed action hooks
*/
function init(){

  foreach( array( 'show_user_profile', 'edit_user_profile' ) as $hook )
		add_action( $hook, 'ShowMemberSince\add_custom_user_profile_fields', 10, 1 );

}

/**
 * Output table
 * @param object $user User object
 */
function add_custom_user_profile_fields( $user ){

	$table =
	'<h3>%1$s</h3>
	<table class="form-table">
		<tr>
			<th>
				%1$s
			</th>
			<td>
				<p>Member since: %2$s</p>
			</td>
		</tr>
	</table>';

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

	printf(
		$table,
		'Registered',
		date( "M Y", strtotime( $registered ) )
	);

}