Screen sharing with absolutely no chance of control?
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.