Click on the tab of the conversation that you would like to participate in.

TIP: If Skype for Business on Mac keeps opening new conversation for the same person, cluttering your conversation tabs, please see: Skype for Business on Mac opens new conversation windows for the same conversation.

If you are participating in multiple conversations, Skype for Business will display these conversations in one conversation window. You are able to toggle between the different conversations using the tabs on the left side of the conversation window.

Click on the tab for the conversation that you would like to participate in.

Outcome

You should now be able to see the other conversation.

TIP: If you do not prefer all conversations to be displayed in one conversation window, you can modify this configuration under Options.