I found the problem. I dumped mysql log and found out that when the app is querying the database for users. It is looking for user tables in the individual site tables. (eg wp_8_users and eg wp_8_usermeta)
User data is kept in the main or default group of tables of a multisite install. wp_users and wp_usermeta (wp_users and wp_usermeta)
So you plugin would have worked on the main site of a multisite but not one of the sub sites.
I was able to do a search of you code and it looks like you need to update two files
process_flow.php line 402ish the database query should be:
$users = $wpdb->get_results( "SELECT users_1.ID, users_1.display_name FROM {$wpdb->base_prefix}users users_1
INNER JOIN {$wpdb->base_prefix}usermeta usermeta_1 ON ( users_1.ID = usermeta_1.user_id )
WHERE (usermeta_1.meta_key = '{$wpdb->prefix}capabilities' AND CAST( usermeta_1.meta_value AS CHAR ) LIKE '%administrator%')");
and
workflow.php line 175ish the database query should be:
$users = $wpdb->get_results( $wpdb->prepare( "SELECT users_1.ID, users_1.display_name FROM {$wpdb->base_prefix}users users_1
INNER JOIN {$wpdb->base_prefix}usermeta usermeta_1 ON ( users_1.ID = usermeta_1.user_id )
WHERE (usermeta_1.meta_key = '{$wpdb->prefix}capabilities' AND CAST( usermeta_1.meta_value AS CHAR ) LIKE %s)",
$user_role ) );
using base_prefix instead of prefix returns the base tables for a multisite install, where the user data is located.
I just did a search of your code for $wpdb->prefix}users so if there are other places you call for user data it will need to be updated as well. I updated those two locations and things seem to be working now.
hopefully this helps