On Windows / Mac browser
Please ensure you have screen share enabled in the collaboration profiles for that user
Screensharing on Mobile Browsers
Mobile browsers do not allow sharing the screen and so you will need to install the lightweight ShowMe App from the app store. (learn more). So even if you are a licensed user on Blitzz but wish to use Blitzz on a mobile device as opposed to a laptop/desktop browser to send a guest an invite, you can only screen share your device if you download the ShowMe App.
Similarly for your guest (who does not have a user license), to share their screen and perhaps show you another app that is relevant to you, you have to get them to download the ShowMe app, but the good news is they don't have to login or put in a password to get started, but can share their screen right away.
Inside the app, the images below explain how to share the screen. For people using the app, the remote expert can even remotely turn on the flashlight from his/her portal. Of course the guest user using the app can also turn the flashlight on by himself/herself.