You are feeling quite rightly angry right now but give yourself time to calm down. It was a very stupid thing for your flat mate to do but she is allowed to bring people back to her own flat, whether it not you agree with her choice of individuals.
Most likely she will feel stupid, regret it and be incredibly apologetic in the morning. She should also clean up the mess, I know the mess isn't nice however if she cleans it tomorrow it is not the end of the world at all.
You need to have an open and honest discussion with her about your expectations from her. Many people get drunk and do stupid things they regret, in all likelihood she will not do it again. You need to explain that you feel uncomfortable with her bringing guys back, if it hasn't been discussed before then she didn't do anything wrong. However she may agree not to do it again if you explain your concerns.
This is where the sleeping in your bed bit comes in, completely inappropriate obviously. However your flat mate although causing it in a way with her actions may have been totally unaware this was going on. She will probably feel terrible about it in the morning but it was not her that did it. It was her friend, make sure once again you make it clear that this can never happen again.
Although your flat mate is completely in the wrong, it is unlikely that this situation will occur again and was a drunken mistake.