I agree with all the other ladies. It's so hard, but I would just add one other thing. I always remember my older sister arriving back one Sunday having stayed overnight with a boyfriend ( I think she was about 17 - and this is about 30 years ago). My father went mad at her, and as she stomped up the stairs she said " We didn't do anything last night that we couldn't have done yesterday afternoon"
I've always remembered that, because of course it's true. Everyone is right - you need to meet him, form your own opinion of him, know where/how he lives and make sure he knows your daughter's age, but ultimately they're probably already having sex which is why staying over probably doesn't seem to big a deal. The talk you probably need to have is a) about her safety and b) about her having a full blown sexual relationship. She's going to do it if she wants to, so better that you are seen to understand and be supportive so long as her safety - physically, mentally and sexually are not being compromised.
Good luck.