Click on join as a host and you'll be live on stage visible to everyone, Option visible only for organizers.
Check out the list of people available online and hover over anyone's name to invite them to the stage. Once the invite is sent they'll receive a popup to join the stage and choose suitable video and audio options to join the stage.
Click on the share screen button at the bottom and then select the respective Screen / Application Window / Browser Tab to be sharde to start presentation.
Click on the end call button in Red at the bottom, You'll be prompted to end session for everyone, click yes to confirm and no to cancel.