Well, I even took phone numbers or e-mail contacts sometimes, sometimes just didn't act on it then...
Sometimes other things come inbetween or just exhausted from RL or wanting to do other things/ talk to other people before talking to these, and then just putting it all off/procrastinating/not sure how it would all go lol...
It's okay to ask, people can give you a fake # or addy anyway or say, 'Hey, we just met..' and not give it to you if they don't really want to keep in contact...
(You can have a prepared answer like, 'Hey, I know we just met, so if your mama'd be concerned, I totally understand...' or something like that

just try to have a sense of humor about it and be understanding )
Alternatively, you can ask them where they like to hang out (which cafes or bars or clubs) or which concert they intend to see next or such..
(I never really did that but I had it done on me haha

Honestly if I really like someone I prefer to exchange e-mails and/or phone numbers.. as I don't hang out in clubs or such regularly lol)
So even if you ask and they give you numbers, don't expect too much of it, be casual at first and just see if you can maybe get together at another similar concert/venue if they're the kind of people who like going to them..
Understand other things can come inbetween and even if they really liked you they may not call.. So you can call them first.. (Or even if you don't and just keep their number, and meet them again, you can say, 'Oh, I've been meaning to call you, things were just soo busy.. How's things?' and it'll be allright, most likely..

)
So, just ask'em!
You go man!