You can use the built in ThickBox to display the popup by Ajax:
//first add thickbox to your page
function add_thickbox(){
if(is_admin() && (isset($_GET['page']) && $_GET['page'] == "my-plugin-file.php") {
wp_enqueue_script('jquery');
wp_enqueue_script('thickbox',null,array('jquery'));
wp_enqueue_style('thickbox.css', "https://wordpress.stackexchange.com/".WPINC.'/js/thickbox/thickbox.css', null, '1.0');
}
}
add_action('wp_enqueue_script','add_thickbox');
//then in your user list for each user you add the thickbox
//popup on the onclick and pass the info needed (like user id) so eg:
?>
<!-- for each user add -->
<a class="thickbox" onclick="javascript: show_user_info(user_id)">user name</a>
<!-- and then add this function once -->
<script>
function show_user_info(user_id){
var aj_url="admin-ajax.php?action=my_get_user_info&user_id=" + user_id;
tb_show('userinfo',aj_url );
}
</script>
<?php
//so all you have left is to create the ajax processing function :
add_action('wp_ajax_my_get_user_info', 'my_AJAX_processing_function');
function my_AJAX_processing_function(){
//echo user info
//and remember to die;
}