To add your guest's camera or screenshare, simply send them the guest invitation link and ask them to join. Your guest can join from mobile or desktop.
The link will take your guest to the green room where they can choose which camera & microphone to use. Your guest can also click the screenshare logo on the bottom of the screen to share their screen with you.
You can bring guest cameras and screenshares onto your stream from the sources panel on the left hand side of Studio. The screenshare is also able to include audio as well for guests as well!