First, you said you liked this girl, you shared a bed and you messed around with her, despite the fact that you currently have a girlfriend that you have been seeing for two months.
So, what, you mentioned that she is older than you, i'm going to presume then that you wish to have something with this girl, while you are still with your girlfriend?
Did you even speak to this girl, you never mentioned it but i am going to presume that since you shared a bed with her some words were exchanged.
I have to say that i have a rather low opinion of this girl, and of you. I hate to be a b***h, but do you really want to be in a relationship with a girl who was willing to sleep with you, knowing that you had a girlfriend?
And, conversely, does your girlfriend deserve to be in a relationship with someone who is willing to cheat on her with a girl he just met?
What exactly do you want? You can't have your cake and eat it too, it never works out.
You could go out with both girls at the same time, or you could dump your current girlfriend and be with the other girl, or you could stay with your girlfriend and never mention that night, you could also 'fess up to your girlfriend and hope to hell she doesn't kick your sorry ass.
Personally, i prefer the last option, no matter what you decide you should tell her, as there were other people at your friends house, she'll probably find out anyway, also no-one deserves to be treated the way you have been treating her. That and there's the possibility of you getting what you deserve.