From the website
We never know anything about the people who provide the answers. What do you know about the 100 people?
Alexander: That is the big mystery, and no, we don’t have a group of people kept in an underground cellar somewhere. They’re found by an online polling company.
Richard: Among all the millions of other things they ask people, they filter in a few Pointless questions every now and again. They’ve got very strict rules about how you fill them in, so you have to do them timed so people can’t look things up.
Alexander: The point is those who answer can’t know they’re being polled for Pointless. You can’t ‘apply’ to be one of the 100 people, because that would then affect the outcome. Who knows who these people are? I’d be delighted to meet someone who had been asked, and find out if they knew they had been asked a Pointless question.
Strangely enough I did do a survey once with timed questions. Don't know it it was for the show though as I never watch it.