1) It means how many people have a regular login to the system. This excludes anonymous portal logins for your customers and suppliers. You can have thousands of partners (customer, employees, suppliers) and none of them count unless they also have their own login into the system.

2) No. Although modules and services that integrate OpenERP with other systems, including Magento, are available for OpenERP - only certified OpenERP modules can be used with the online offering.

You should contact an OpenERP partner (http://www.openerp.com/partners) in your region - some can help you host your own installation of OpenERP in the cloud that does support this type of integration.