A simple custom control I wrote for the WordPress theme customizer that allows you to create user select drop-down list...

Selected_Users_Control.php

<?php 
if (class_exists('WP_Customize_Control')): 
class Selected_Users_Control extends WP_Customize_Control {

        public $type = 'option';
        public $query = array('orderby' => 'nicename');
        public $description = '';

        public function render_content() {
            $query = $this->query;
            $pgs = $this->user_array($query);
            ?>
            <label>
                <span  class="customize-control-title" ><?php echo esc_html($this->label); ?></span>
                <select <?php $this->link(); ?>>
                    <option></option>
            <?php foreach ($pgs as $key => $value): ?>
                        <option value="<?php echo $key ?>" <?php echo ($key == $this->value() ? 'selected' : '') ?>>
                        <?php echo $value; ?>
                        </option>
                        <?php endforeach; ?>
                </select>
                <span style="display: block"><?php echo esc_html($this->description) ?></span>
            </label>
            <?php
        }

        public function user_array($args = '') {
            

            $arrray = get_users($args);
            foreach ($arrray as $items) {
                $pgs["{$items->ID}"] = $items->user_nicename;
            }
            return $pgs;
        }

    }
endif;

?>