To start your secure screen sharing session, just click on the person you want and then click on 'Show'.
To add more people into the session click on 'Add' and select the people you want.
If your colleague is on Yuuguu - they will be asked if they want to join your screen sharing session. They can choose to 'Accept' to see your screen or 'Decline' if they don't want to. If they are on another IM network they will be sent an instant message with a hyperlink in it. They click on the link and will be able to instantly see your desktop in their browser. There is nothing to download or install at their end.