To share Office 365 with her customers HCS uses a feature called "Azure Active Directory Business to Business". This means that an HCS admin follows the following steps to grant HCS Clients access to SharePoint:


  1. Add the user as a Guest User to Azure AD 
  2. Add the user to the applicable SharePoint Site
  3. Share the address (url) of the Site with the users requiring access


Step 1 results in an email invite for which the user needs to follow some steps:


After these steps have been completed the administrator of the SharePoint Site will add you to the site and share the address of the site with you.