If you are at the front of a group, or are in charge of a gathering, whatever you say in a loud enough voice would be addressed to the whole group. When you have everyone's attention, all you need to say is "please be seated" or "please take your seats" and the whole group would know you are talking to the everyone.

You could say, "Please be seated" or "Everybody please be seated". Note "Please all be seated".

"Please be seated" is appropriate. We don't usually say "Please all be seated"

Please, all be seated is used when you are addressing a group or an audience. The other refers to a one or two people.

If I'm doing a presentation and I'm ready to start, I'll probably say something like, "Will everyone please be seated? We're ready to start. The techies have fixed the problem." (Or, "Will everyone please take their seats?") If I'm an usher at a theater and the play is ready to begin, I might approach any audience member who is still standing around and say, "Please be seated. The play will begin shortly."