ok, in that case a curfew wont help. she'll just ignore it as there isn't really anything you can do to enforce it is there.
keep the lines of communication open, keep conversations relaxed. as hard as it may be dont let her see that you are itching for her to stop meeting these men as that could just act as an incentive to do it more.
do you think she is doing it to test the boundaries or for attention or just because she really likes these men? is she getting very drunk when she is going with these men?
i was living on my own at 19 and my parents wouldn't have known where i was for weeks at a time. (i was just being normal, working, having friends over, the odd night at the pub but for all they knew i had fallen off the face of the earth)
the thing that made me more likely to get in touch or visit them was when they eased off on the phonecalls asking me to call and saying they were worried about me. i can see now that it was selfish and inconsiderate of me not to call them back but at the time i was so pissed off with them thinking i wasn't capable of keeping myself safe. when they showed that they had backed off a bit, i felt more ready to talk with them and let them know all was ok.
i know it's hard. my mum still worries about me constantly and i've been a mum myself for almost 8 years now!
give your girl some credit, yeah she'll make mistakes and might possibly end up in bad situations but if she's determined to go there, she'll do it anyway.
in your shoes i would just let her know that you are there for her 100%. i would also try (i know it is easier said than done) to relax. let her feel relaxed enough around you to share hopefully as much as possible so that you know what's going on with her. maybe start sharing some stories of your own youth, let her see that you were young once,. have a laugh about the things you got up to, bond over it and let her know that your ok to talk to if she feels like ranting about her latest boyfriend or what her best friend is doing with such and such behind X's back. let her enjoy your company as more than just a parent figure. the more she relaxes with you the more she'll trust you.