I was wrong about the accomplice...looks like he acted alone...
“....gunman bought a ticket to the Batman movie, sat down for about 15 minutes, and then propped open the back door to the theater when it was dark. The gunman then went back to his car and parked it in the back of the theater, grabbed the guns and went in through the open back door.”
http://www.thedenverchannel.com/news/31289126/detail.html
Well at least he had the common courtesy to actually purchase a ticket.