Consider the following scenario:
You have Microsoft Lync 2013 installed on your computer, however, your Lync account is not provisioned for Lync 2013.
You receive a Lync meeting request in Microsoft Outlook.
You try to join the meeting by clicking the Join Lync Meeting hyperlink.
In this scenario, you cannot join the meeting when Lync 2013 starts automatically and prompts you for the credential. Additionally, there's no option to join the meeting by using Lync Web App.
To resolve this issue, install the following update:
2850074 Update 2850074 for Lync 2013: June 2014