The pop-up that comes up when you click "to" should have two boxes, the top one has your contacts, and once you've highlighted a name, it should have a check next to it in the top box, and the name should appear in the bottom box. Once you have all the contacts that you want to send the e-mail to selected, just click done at the bottom left of the pop-up, and you'll be returned to your compose mail screen with those addresses included in your "To" box.
On edit: There is no "two" box when you compose a message, I'm not actually an idiot and I do know the difference between 'two' and 'to'