There is no password-protected or private channel where you can control who can join. But you can use some hard-to-guess channel names or even GUIDs like “81df86c0-6fb5-4bf3-b6e8-14f5d530edad”, reducing the possibility of someone ending up there.

There is no hard limit set on the number of users who can join the channel. Since each user who joins the channel is streaming video to you, it can get laggy with more concurrent users. It should work without any problem for ~ 8 users.

Yes, you can embed any Hello channel to your website using iframe. You can find the instructions here.