this actually came up the other day with me and my buddy
while i have never had the official "now we are together" talk with a girl, he had with his gf's
for me it just happened and wasnt really an issue to make official
so we thought what is the clear defining moment when it crosses over?
our answer- the first time she spends the night and you dont get laid, then you are officially something, otherwise its just booty

as far as a way to ask her out, dont overdo it, high expectations at first leads to dissapointment later
a buddy of mine just claimed the girl his gf in front of his friends randomly, she was a little shocked but very impressed that he took charge and didint give a fuck what they thought, needless to say he got his twinkie wet that night
you know her dude, just go with the flow, she will dig you no matter what