The multiselect field uses the same options as the native checkboxes and select fields. Without making any changes to the field, you could use the query API to fetch a list of users as options.
An alternative would be to fork the field and set the options to users. A simple way to achieve this would be to change the constructor in /field/field-multiselect/field/multiselect.php
like this:
public function __construct() {
$this->icon = 'chevron-down';
foreach(kirby()->site()->users() as $user) {
$this->options[$user->username()] = $user->username();
}
}