Add a custom column in users list page

One thing you should remember is that function, which will be hooked into manage_users_custom_column action must have 3 parameters, the first of which (i.e. $val) should be the returned value:

// Add custom column using 'manage_users_columns' filter
if(!function_exists('bm_utm_column')){
function bm_utm_column($columns) {
    return array_merge( $columns, 
        array('utm_source' => __('UTM source'),
            'utm_medium' => __('UTM medium')
            ) 
        );
    }
}

// Add the content from usermeta's table by using 'manage_users_custom_column' hook
if(!function_exists('bm_utm_column_value')){
   function bm_utm_column_value($val, $column_name, $user_id) {
    if ( 'utm_source' == $column_name ) {
        //Custom value
        $val = get_user_meta($user_id, 'utm_source', true);
    }
    if ( 'utm_medium' == $column_name ) {
        //Custom value
        $val = get_user_meta($user_id, 'utm_medium', true);
    }
    return $val;
   }
}
// Hook into filter
add_filter( 'manage_users_columns', 'bm_utm_column' );
add_action( 'manage_users_custom_column', 'bm_utm_column_value', 10, 3 );