Add WordPress users to a custom post type

You can use the ACF plugin for https://wordpress.org/plugins/advanced-custom-fields/.

Steps need to do:

1. Download and installed ACF plugin.
2. Click on Custom Fields link at the left side menu
3. Add custom field as:
   
4. Select custom as and publish it:
   
5. Now add this code in functions.php file as:

function acf_load_color_field_choices( $field ) {    
    // reset choices
    $field['choices'] = array();
  $blogusers = get_users();
  $field['choices'][0] = "Select User";
  foreach($blogusers as $user){
            // append to choices
            $field['choices'][ $user->ID ] = $user->display_name;
    }
    // return the field
    return $field;    
}
add_filter('acf/load_field/name=select_user_for_post', 'acf_load_color_field_choices');

6. Now if you create Groups post you will see users list in a dropdown as:

Important details:

1. get user list

2. Dynamically populate a select field’s choices