Note that admin permissions in Office 365 are required to enable this feature.
1. Login to admin at http://portal.office365.com
2. From the left-hand menu, click 'External Sharing' and select 'Skype for Business' from the drop-down menu.
3. Check the box that says 'Let people in your organization use Skype for Business to contact people in other organizations'.
4. (Optional: If there are any domains that you don't want to allow access to via Skype Business, type in their web addresses. It is useful for when you also want to make sure Skype for Business isn't used to contact people in certain businesses who are also using Skype for Business.)
5. Under 'Public IM Connectivity', check the box that says 'Let people use Skype for Business to communicate with Skype'. This is what enables you to use Skype for Business to connect with people using free-to-use Skype (e.g. for personal use).
6. Click 'Save' to finish.