Consider the following scenario in Microsoft Skype for Business 2016:
User1 starts a conversation together with User2.
User1 presents programs together with User2.
User1 invites User3 to the sharing session.
User1 checks the Give Control options.
In this situation, User2 (the original user) is unavailable and un-clickable in the Give Control options.
To fix this issue, install the January 2, 2018, update (KB4011623) for Skype for Business 2016.