Welcome to the forum.
It should be *possible*, but will not be easy to accomplish.
The biggest hurdle would be to separate the people cleanly, for the background of the original Video. If they had been shot of greenscreen, or similar, it would be easier. Then, all you would do is Key out the greenscreen, then use the Fixed Effect>Motion>Position, to place duplicates of that Keyed footage over a background - sort of like the Orc hoards in LOR - well, sort of.
Knocking out, or Keying out the background from your Duplicate Clips will be the task, and it might be insurmountable. Now, Adobe After Effects has a great tool for that, the RotoBrush, which even has Content Aware capabilities, in the CC (Creative Cloud) version: http://blogs.adobe.com/aftereffects/2010/04/roto-brush-in-after-effects-cs.html
Maybe others will have ideas for using one of the Keying Effects, or VideoMerge in PrE, but I just see little but problems.
If you could also post a screen-cap from your Clip, so that others can see the background that you are working with, it might be helpful.
Thanks for the information. We have shot the individuals against green screen so we might be able to do as you suggest. I think we may have to look at getting After Effects as this, as you say, may be easier. We are a school with children from 4 to 11 years old so we need to keep it as straight forward as possible. Thank you so much for your time and expertise.
If you DID shoot on a greenscreen, then you are quite close, with a lot less work to do.
First, what OS version, are you running PrE 11 on? The reason for this question is to point you to which Keying Effects to experiment with. The Windows version has more, that the Mac version lacks.
In the PC version, there are several Keying Effects, with greenscreen, being the first that I would look at. If you can get a good Key of the actors, then it just a matter of Duplicating that Clip, placing the Duplicates on higher Video Tracks, and adjusting them with the Fixed Effect>Motion>Position, to get them, say beside the original. Repeat with each Keyed Duplicate. Depending on what you want for a background, for your "crowd scene," I would put the Clip of the background on Video Track 1, with the original Clip of the actors on Video Track 2, then the Duplicates, Positioned as needed, on Video Track 3, 4, and on.
Hope that this helps, and if you can let us know your OS version, someone can point you to the Keying Effects, that will likely yield the best results for your Project.
Thank for the further information. I am running PE11 on Windows 7.
I will have a look at what you suggested. We do most of our shooting against a green screen as we have our own green screen studio in school.
Many thanks again.