With Live Present, you have the option of creating a one-time access code that you can send along to all viewers. This helps add increased security around your presentations and won't let people that have your presentation URL drop by unannounced.
You can choose to use an access code when starting a Live Present session from within a document:
or after selecting Present from the sidebar:
If you've chosen to have an access code, here's how your visitor will be prompted when they first enter your presentation URL: