Hi @mmn
To achieve this select field, you can populate the choices dynamically following this tutorial:
http://www.advancedcustomfields.com/resources/tutorials/dynamically-populate-a-select-fields-choices/
Firstly, create a select field,
Next, add the filter code to populate the choices of the $field
Jump on google to find the WP code to get a list of all user roles
Loop through the user roles and append them to the $field[‘choices’]
Easy
Thanks
E
