If the guests see the mouse cursor moving around on an all black screen, or a black rectangle where an application window should be, it is usually caused by a firewall or security software that is interfering with sending the presenter's screen image to the servers.
This has been seen in the firewall of Webroot SecureAnywhere as well as some other security software solutions.

To test the security software to see if it is the cause, disable the security software and then exit/quit the GP5 application.
Then re-open the gp5 application while the security software is still disabled and join the session again to see if the problem persists.
If that resolves the issue then you'll need to find out where in the security software you can allow GP5 to run without the security software inhibiting it.

There are usually 'white lists' in security software that allow you to add a program so the security software does not try to alter anything as it is running.
Please contact the manufacturer of the specific security software for technical support on configuring their software.