Consider the following scenario:
You create a Microsoft Skype for Business meeting.
You set the These people don't have to wait in the lobby option on the Skype Meeting Options page to Anyone (no restrictions).
Some users dial into the meeting, and then wait in the lobby.
You join the meeting through a Skype for Business client.
In this scenario, the users continue wait in the lobby and don't join the meeting automatically.
This issue occurs because the MAPI property of the meeting item in Microsoft Outlook is old.
To fix this issue, install the August 2016 cumulative update 5.0.8308.965 for Lync Server 2013, Front End server and Edge server.