What would you have done in this situation?

I'm sitting with my friend at a smoking gazebo/bench area on campus at uni and two girls come up and sit down, one on each bench. We're sitting facing each other. The same girls had come down for a smoke 30ish minutes earlier and we talked a bit, but I was tuned out for most of the convo. The second time they came down we talked about police and security on campus, gun laws, politics in US vs. Canada. I was into the conversation this time. The girl who sat down next to me kept agreeing with what I said (until we got into political talkin'), there was some good eye contact, and she complimented my awesome peruvian hat. Later she says something to the effect of "I'm not into big guys," while looking at my friend, "I'll keep my nice scrawny boys," then over to me. At the time I realized she was hitting on me, but we just continued the conversation without acknowledging it. Afterwords my friend asked me if I knew she was hitting on me, and said I should have asked "What you doing tonight?" and tried to go off with them. And in writing this I realize that he was probably right, but it didn't even cross my mind at the time.

When he talked about it he acted as if it were a practically a free pass to continue the night with them, but I only saw it as some flirting and didn't act on it.

You always gotta have that kinda stuff on your mind man, or youll miss out and regret things like this.
I would have at least number closed, with how she seemed to be spoon feeding you maybe even show her your awesome new bedsheets XD
When you dont escalate, your a puss, and they could even think of you as gay; theres almost never a bad time to escalate when your alone with a girl, but if you dont your never going to.

I'm just shocked that I didn't act on it, even while I realized she was hitting on me. She's on campus so I'll see her again, and be on my toes this time. Looking at it a few hours later I already feel like it should have been obvious what to do.

maybe you were not that interest, or else regardless of whether she gave you some IOI you'd be trying to get hold of her.