Once you've entered the Yostream Studio, you'll see the following screen.
At the top-right corner, click Invite Guests.
Click the Copy Link and send it to the guest(s).
As soon as someone joins in by clicking the invitation link, you can see her/him in the left panel of your screen.
Once you toggle, the guest will start appearing on your canvas.
You've successfully added a guest to your live stream. With the same approach, you can add many more (depending on your pricing plan).