RobE asks about using Active Directory Replication when you have a mix of internal and external customers.
You can still use replication in these circumstances, and have two choices.
The first thing to understand is that if you switch on replication and then create a customer record using SerioClient, you can still use that customer - for Incident logging. When replication kicks in records like this will remain untouched. So that means your customer database can be a mix of both replicated (internal) and non-replicated (external) customers.
Your other option is to store your external customers as Contacts (yes, Active Directory has such a data storage concept). Serio will replication from your Contacts, even though Contacts are not domain users.