Your best bet is going to be to mix the input from multiple microphones. If you get a simple mixer, you can have them all combined and not have to match it up after the fact. You would just leave all microphones turned up and which ever one has someone talking would play through.
You could also potentially use a really sensitive mic with a device called a compressor. This would prevent the louder people from being too loud while still having enough sensitivity to get the quieter people. It will be a cheaper than a mixer and multiple mics, but it is also going to be much noisier.
You really don't have any option other than to get a new camera that can support one of the two options or to try and record audio separately to something like a Zoom H4N and then combine it on a computer after the fact. There is no magic trick that is going to allow you to use such a limited camera in the context you describe.