I'd like to make an slightly off topic comment which some on here probably won't like. Few professional photographers will touch concerts with a barge pole. Hundreds of kids who would like to break into photography shoot concerts for free, some of them are better than others. I beg you, your images are vaulable, the kit to shoot them costs a lot of money, do not give away your images for free, not for any amount of stupid worthless excuses like "it'll help your portfolio" because it won't. When the groups or the venues start letting you in for free then they might value your work, but until then your just another free image source.