I've stumbled upon a bizarre problem with Skype for Business 2015 (the client application, previously known as Lync 2013 but automatically upgraded to SfB 2015 by some Office update); maybe I'm missing something, but I'm unable to find a solution, hence this question.
The issue: someone invites you to a meeting, but you don't have a Lync/SfB account, thus you need to join the meeting as an external guest by clicking on the meeting link you received; however, the application is actually installed on your computer, as part of the Office 2013 package; in this scenario, clicking the meeting link will automatically launch the application, but then it will get stuck on asking you for sign in information, which you don't have. It looks like there is no way to avoid launching the application if it's actually installed, or to sign in as a guest.
How can you join a Lync 2013 / Skype for Business meeting as a guest when the application is installed on your computer but you don't have any account you can use?
Best Answer
It is a little hackish, but if you right click > copy the Join Skype Meeting url from the invite, paste it in your browser and add "?SL=1" (without the quotes, so it would look something like https://meet.yourdomain.com/user/98daf9w?SL=1) on the end, it will force to join in via the web app (where you can join as guest) instead of automatically opening Lync/S4B client that requires a login.
Hope this helps!