Screen sharing with absolutely no chance of control? I'd like to give a presentation to other people who happen to be in the same room. I'll be running various programs and would like my screen to be visible as I work. Instead of a projector, I am wondering if I can use screen sharing. It's not clear to me if there is a way I can start such that I can be absolutely sure there is no chance of others having any control over my computer, cursor, command-line, etc. I only want to broadcast my screen to other computers' screens instead of using a big monitor or projector.

How exactly can I set up screen sharing to be sure? Are there good alternatives within OSX? (I don't want to have to download or install 3rd party software).
There is nothing built into OS X that allows you to share your screen with multiple people at once.

One-to-One : Screen Sharing

OS X's built-in Screen Sharing is limited to a one-to-one connection. Screen Sharing is based on VNC and can be limited to observation only.

One-to-Many : Google Hangout / Skype

Consider using a Google Hangout to share your screen. While it does require third party software, the use of the Chrome browser is free. This approach also ensures no-one can control your shared screen.

Alternatively, Skype group calls allow for multiple people to observe your screen.