SharePoint Online provides cloud base SharePoint portal to the subscriber. Whether you subscribe Enterprise plan that allow many internal users or Small Business plan starting with 1 to 20 users, you always be able to invite external user.
This scenario is very useful when you build a start-up company and decide to use SharePoint Online for your portal. You can start with very cheap subscription of 1 user as the company administrator, and invite other staff as external user to the site. The external user will be able Contribute to the portal, devoted as Site Collection Administrator or as simple as ordinary reader – the only limitation is they need to buy separate Office client license to work with Microsoft Office.
Following video shows how to enable external user access in SharePoint 2013, SharePoint Online and O365.