Introduction
Click on the "Participants" button and then click Invite More People. Find a contact to invite, and click OK.
Instructions
1) In a conversation already in progress, click Participants.
2) Click Invite More People.
3) Pick an existing contact of yours, or type a name into the textbox. Click on the correct contact to select it, and then click OK.
Outcome
The person you invited to join the conversation will be notified and prompted in Skype for Business to join the conversation.