Lync Attendee is a conferencing client that allows non domain or external users to fully participate in Lync Server meetings including video conferences.
The Lync web app does not allow Video, only audio.
Microsoft Lync 2010 Attendee – Admin Level Install
Lync 2010 Attendee Hotfix KB 2752160 (Admin level install)
Microsoft Lync 2010 Attendee – User Level Install
Security Update for Microsoft Lync 2010 Attendee (Admin level install) (KB2726388)