Like most things, this is an area of prudential judgment. There is nothing inherently immoral about a group of men and a group of women sleeping in the same house.
But, prudential judgment might lead you to the conclusion that a particular sleepover is unwise and likely to result in poor judgment, bad behavior, etc.
Factors to consider include the purpose of the sleeping arrangements-- i.e., could you go home to your own place as the night wears on? Why would adults be sleeping over when they are in their own town with access to their own homes? Is the purpose of the sleepover due to alcohol and not wanting to drive? (there is a clue that it is headed in an immoral direction-- planned drinking til drunk)
Or is this a camping trip or a trip to the lakehouse, etc, where a group of people are spending a weekend many hours from home?
I think people on this site are either overly scrupulous or overly lax. Really, this is one that could be perfectly fine or it could be trouble with a capital T. Only you know the people involved, the situation, how people will be segregated for sleeping, etc. There isn’t anything wrong with men and women going in groups to do things like camping, retreats, lake house or beach house rental for a weekend, ski trip, etc. But, again, it is all in knowing that the people are of good character and plan appropriate sleeping arrangements.
So, form your conscience and use prudence.