Andy Hughes was paid, by the company behind AB, to set it up. Which means that he was certainly the first member. (That bit's fact, because Andy provided the same information himself recently. The following is conjecture)
Since Andy presumably needed to 'seed' the site with some questions to get it started he almost certainly asked the first question. Further, it's quite likely that he wanted the site to have some answers on it as soon as it 'went live', so he might (under a different username) have also provided the first answers as well.