I think it really depends on the type of relashonship you and the girl share. If you are already together, she should assume you two will attend as a couple already. If you and the girl are on a friendly basis and you know she isnt going with anyone, then it's best to ask who she is going with... When she says "No-one," or "just my friends" ask her if she would like to go with you. Lastly, if you and the girl don't speak, or she doesnt know you... you should ask her friends, or people that know her if she has a date. If she doesnt, I would start taking to her a little more, and try to get to know her for a few weeks before you ask.
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There are sooo many cute ways. just think of something that going with how he asked you. Im sure if he asked you he would love an answer... (: