Xero's subscription model helps them grow sustainably. They have trained and positioned Xero experts around the globe. The experts serve a region and sign up subscribers underneath them. By targeting professionals who may already have a client base that is in Xero's target market (for example, CPA's who advise startups or Bookkeepers who may be searching for something more user-friendly than Quickbooks) they have been able to scale quickly. It helps that their user interface is intuitive, as well.