If you're using an Apple computer, you may run into trouble using video, audio, and/or screen sharing for the first time after you've downloaded the Zoom app.
1) Once you've downloaded Zoom, go to System Preferences and select Security & Privacy:
2) Switch to the Privacy Tab and click the lock icon on the bottom-left, then enter your credentials to allow changes:
3) On the left, select Camera and allow zoom.us by adding a checkmark next to zoom.us on the right:
4) Repeat step 3 above for the Microphone and Screen Recording options on the left.
When you're done, click the lock icon again to lock in your changes.