1) In the left-hand menu, click Chats. Then, choose the desired conversation by double-clicking it.

2) Click the "Add Participants" icon.

3) Click on the Invite more people text box and type the name or email address of the contact that you would like to invite. Then, double-click on the desired contact to add them to the conversation.

OUTCOME

The person you invited to join the conversation will be notified and prompted in Skype for Business to join the conversation.