One of the key enhancement in SharePoint 2013 is the ease of sharing. Being able to share a site, library, or piece of content is as easy as clicking on the Share button and specifying a user and assigning the rights you want them to have. This, along with the new licensing model that allows non-organizational users to use your SharePoint environment without requiring a CAL or FIS licensing for the farm, have made SharePoint 2013 target for extranets.
Once a site collection is established sharing with external users is quick and easy. The one thing that is that upon intial setup of a private site collection in SharePoint Online is to setup sharing with external parties. By default sharing is set to “Don’t allow sharing outside your organization”. When you attempt to share with outside people from a site collection that still has this set you get a message that no results are found when typing in an email address.
To change this you need to log into the SharePoint admin center select the site collections tab on the left nav and then select the site collection you wish to enable external sharing for.
Once selected click on the Sharing option in the ribbon and set the option to “Allow external users who accept sharing invitations and sign in as authenticated users”. This will allow you to share securely with specified users.
Once this is set return to your site collection and attempt to share again. You should find a result based upon the email address that you entered now.
If you prefer to allow users to share links and allow external parties to access the site collection without having to authenticate you can chose the “Allow both external users who accept sharing invitations and anonymous guest links”.