You need to register to use the app, it takes a minute and right away you can using Yugma. Create a meeting to and invite attendees for your meeting. When you create a meeting, a free teleconferencing and number are generated for you. When you start your virtual meeting it will ask you download the desktop app, install this app to get started.
If you are the presenter, invite your contacts once they are in you can start sharing your desktop, share files etc. There is a chat option to chat with your guests, if you want to chat with one guest privately you can use the private chat feature. You can change the presenter easily using whiteboard tools.
Yugma has Skype integration, if you are Skype user you can use this to invite your skype contacts for collaboration and sharing.
Also check out Yakkle, another screen sharing tool.