In April 2019 Microsoft discontinued the Skype for Business Web App for Mac, offering guest access to meetings through the desktop client.
The desktop app brings many advantages over the Web App: Screen sharing, audio, and video are significantly more reliable.
To join a meeting as Guest:
Download the Skype for Business Mac client: https://go.microsoft.com/fwlink/?linkid=831677&clcid=0x409
Click on Join Meeting as Guest
You will now be able to enter the Meeting URL and your Guest name to join the meeting.