Its easy to setup a read-only user that can only see a limited set of monitored servers, probes or dashboards. Just tag your system or probe widgets (on the systems or probe tab) or your custom dashboard with a unique tag, specific to that user. Keep it simple, perhaps c222 for customer number 222.
This is a great feature for giving access to internal and external customers, to view only their stuff.
Simply add their designated tag as a restriction when setting up the new user.