Do participants need to download anything?

They would need Flash Player 11.2 or newer, but that is all. System requirements for attendees are:

Windows

• 1.4GHz Intel® Pentium® 4 or faster processor (or equivalent) for Microsoft® Windows® XP, Windows 7 or Windows 8
• Windows 8.1 (32-bit/64-bit), Windows 8 (32-bit/64-bit), Windows 7 (32-bit/64-bit), Windows XP
• 512MB of RAM (1GB recommended) for Windows XP, Windows 7 or Windows 8
• Microsoft Internet Explorer 8, 9, 10, 11; Mozilla Firefox; Google Chrome
• Adobe® Flash® Player 11.2+

Mac OS

• 1.83GHz Intel Core™ Duo or faster processor
• 512MB of RAM (1GB recommended)
• Mac OS X 10.7.4, 10.8, 10.9
• Mozilla Firefox; Apple Safari; Google Chrome
• Adobe Flash Player 11.2+

Linux

• Ubuntu 12.04; Red Hat Enterprise Linux 6; OpenSuSE 12.2
• No Add-in support for Linux. Users on Linux can attend meetings in the browser.
• Mozilla Firefox
• Adobe Flash Player 11.2+

Mobile

• Apple supported devices: iPhone 5S, iPhone 5, iPhone 4S, iPad with Retina display, iPad 3, iPad 2, iPad mini, and iPod touch (4th & 5th generations)
• Apple supported OS versions summary: iOS 6 and higher
• Android supported devices: Motorola DROID RAZR MAXX, Motorola Atrix, Motorola Xoom, Samsung Galaxy Tab 2 10.1, Samsung Galaxy S3 & S4, Nexus 7 tablet
• Android supported OS versions summary: 2.3.4 and higher