1) Sign in to Skype using a Microsoft account. (To connect with Skype for Business users, you cannot sign in using a regular Skype account; it must be a Microsoft account.)

2) Click the icon of a person with a plus symbol (+), and then search for the Skype for Business user by typing their email address. Once you locate them, click Add to Contacts.

Outcome

The Skype for Business user will be on your Skype contacts list, allowing you to place voice calls to them or send/receive instant messages.