they need to download it yes. Jitsi is a bit of a dog, causing frustration for all parties until it works, then it's fine. If you want pick up and play then probably google instant meeting is about the best. Zoom is a bit split into a web version and a binary, neither of which I trust very much, but it works. My appeal to credibility here is 25 years researching telepresence systems. It galls me to land on google as I run everything degoogled, but your use case is very pick up and play.