Good practical question; the program is altered quite often, but as of about 06-30/07-01-2020 there wasn't a toggle to do that. The meeting host must adjust permission for participants to join or for particpants to initiate screen sharing etc: there's a small green up-arrow just above & to the right of the green center control for screen sharing which lights up matching green; selecting that up arrow opens a box which has some settings, and also for Advanced Settings: select Advanced Settings opens a box which contains round blue alternative toggle buttons, among those, allow/ disallow participant screen sharing, and allow/ disallow participant participant initiation of screen sharing, and whether only one screen can be shared at a time, etc: the screen sharing is a subset of vid sharing options, so vid is first: screen sharing is a vid customisation option cf a separate alternative from vid.
Setting the toggles to, eg, authorise participant screen sharing/ participant intiation of screen sharing, then exiting Advanced Settings, & exiting Settings then results in the authorisations for screen sharing / screen sharing initiation by participants, and the little up-arrow being a muted grey colour: and it also may reset of other toggles/ processes/ functions, so good to glance at other features to check if any other settings have also been changed, & if so perhaps, have been changed and might be preferred to be set to where they were set previously!
The throughput observation is significant, since the program uses about 9x as much throughput for the vid as the program uses for audio!
So, maybe simply ask participants to switch vid to Off to reduce machine usage intensity: the audio uses about 1/9 of the bandwidth that the vid uses