Truly, not a lot! At minimum, all they need is a device with a camera and microphone (a laptop or smartphone, for example) and a decent internet connection. You should double check that your guest's device meets Rivet's current system requirements - most do - and then have them download the Rivet App from Rivet.Stream or their device's app store. There is nothing more that your guest needs to supply!
Check the System Requirements below: