How to add a regular Skype users to Skype For Business
It applies to Skype for Business for Windows.
1. In the Contacts view, click the button with Add Users icon -> Add a Contact Not in My Organization -> Skype.
2. Enter the Skype user name of the person you want to add in the search field.
3. Once you find the Skype user, right-click on the user name > Add to Contacts List > select a group you would like to add a user to.
4. You will see a notification that a contact request was sent.
Note: The new user will have pending status in your contacts list and you will not be able to communicate with this user until they accept your request.