I was lead to believe it was actually 5. I think you pre-select 5 people and chose a category for each, like music, sport, history etc to give you a bit more of an option. Could be wrong though! :)
I thought it was five as well. I heard that you pick your 5 people, then a millionaire representitive goes round to each of their houses to check they dont cheat by looking in books or the internet whilst they are helping.
Both answers are right......you pick 5 before the show then, depending on the question that you want to use that lifeline on, you select the person you think will be best able to answer out of your 5 picks.