Is this for an animation, or still? Should the people be visible up close, or only from distance? The more details, the better. :)
One solution would be getting good-looking models, converting them to proxies, and then placing them on the chairs/wherever they should be, using some automated system - for example the new Corona Scatter, which is capable of scattering in regular patterns, on splines, etc. Corona MultiMap could be used to randomize clothes, skin color.
These are just suggestions. The execution would probably require more tools and more careful planning.