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However, if the joining user does not have the Lync client installed, they can use the Lync Web App - the cloud based Silverlight enhanced web application which runs within the browser.
Here you can see the Lync Web App loading (it doesn't take very long for a very comprehensive and feature heavy application) and how it appears on the Lync client application on the desktop.
Though the Lync Web App is ideal for anyone ('guests') outside the company or enterprise to access a conference call or shared collaboration service, it can also be used for those within the company who are working from home or do not have the Lync client installed. Either way, it's merely another access point to gain entry into the service.