Here is a good timeline. There is more to the article but I'll post the gist of it:
http://rockvillemag.com/home3/2009/12/17/metallica-fan-morgan-harrington-still-missing/
... Tragically, due to circumstances still unknown, Harrington ended up outside of the venue before Metallica even took stage around 8:30 p.m.
The facts minus the sea of web conjecture are as follows:
Harrington arrived safely at the arena in her car which was driven by a friend. She was in the company of two girlfriends and a young man who was the boyfriend of mutual friend. It was in fact Harringtons car, not a friends vehicle as earlier reported in the media, which did take all four students to the arena. The keys remained with Harringtons friend, inside the venue.
Harrington made it safely inside the JPJ arena. Around 8:15 to 8:20 p.m. Morgan parted from her friends to find a restroom.
Around this time, Harrington was reportedly seen with a minor facial cut. Virginia State Police believed the cut was the result of a trip or fall, not an assault.
Harrington ends up outside of the JPJ arena and is not allowed re-admittance as per the standard JPJ arena policy.
8:30 p.m. (estimated) Harrington speaks to her friends (on her cellphone) who are still inside. Morgan informs them she was outside of the arena and was not going to be let back in. She speaks of her intent to find another way home or would possibly meet up with them after the show.
Over the next hour, eye witness reports place Harrington in the surrounding area of the JPJ arena parking, then moving further into the UVA campus.