[list_group_users]

list-group-users.php

<?php
add_shortcode( 'list_group_users', 'list_group_users' );
function list_group_users( $atts, $content = '' ) {
	$output = '';
	if ( isset( $atts['group'] ) ) {
		if ( $group = Groups_Group::read_by_name( trim( $atts['group'] ) ) ) {
			$group = new Groups_Group( $group->group_id );
			$groups_users = $group->users;
			usort( $groups_users, 'sort_list_group_users' );
			if ( !empty( $groups_users ) )  {
				$output .= '<ul>';
				foreach ( $groups_users as $groups_user ) {
					$output .= '<li>' . wp_strip_all_tags( $groups_user->user->user_login ) . '</li>';
				}
				$output .= '</ul>';
			}
		}
	}
	return $output;
}

function sort_list_group_users( $u1, $u2 ) {
	return strcmp( $u1->user->user_login, $u2->user->user_login );
}

Copy the file to your active theme's root folder and in its functions.php add this line at the end:

include_once 'list-group-users.php';

Example shortcode usage:

[list_group_users group="Registered"]