As soon as they have had the full course of injections. This is normally at about 13 weeks. It may also depend on the ringcraft. I took my puppy at 13 weeks, but her brother had to go to obedience class for a week before he was allowed to join us. Still don't quite understand that one.
Lindylou, different clubs are different, at our RC we have had 9 week old pups that come up only to be cuddled by many people and get used to the noise, however the only people we let take advantage of this is people we know, and know they want the best for their puppies!