Drupal autocomplete form, multiple values
Add a select list to your form. Add some #ajax to your textfield. Every time the textfield changes content, add the value (via #ajax) to your select list. Use the values of your select list in the submit callback.

OR

You could to it by attaching a custom JavaScript file using $form['#attached']['js'] with some good old JavaScript to fetch the users and add them to the csv textfield (autocomplete). Then split the values in the submit and add them to the database.

OR

Just use a select if that's OK with you.