This issue must be specific to users on Windows 10.
For the first time when you Click the option "Email an Invitation", a dialog box appears as displayed below.
Ensure, you Choose Outlook (Desktop), if you have configured Outlook Client as your Mail Client.
If you have failed to choose your default Outlook Client follow the steps below to reset and re-open Lifesize Cloud Desktop client.
Click Windows Log > Settings > System > Default Apps > click Reset