This certainly won't apply to spaces that pre-date modern acoustics (for example, cathedrals), but I believe it's been standard practice for a while now to fit concert halls with seating that has similar sound-absorbing characteristics to a human body, such that the size or distribution of the audience shouldn't noticeably affect the sound of the hall.
It would be interesting to find out which halls that have been sampled actually have this property, though.
@steff3 said:
However I think it is pretty much impossible to make IRs with human beings in the space.
I don't see why, other than the logistical complications of getting a load of people to sit there quietly during the sampling process (although removing any squeals they make when bangs go off could be amusing).