TMZ broke the story ... Demi was out of town when her caretaker threw a party for a small group. Most of them left to go out at some point in the evening, and when they returned they found a 21-year-old male who stayed behind facedown in the pool.

Law enforcement told us the victim had told the group he couldn't swim and apparently tripped over something while everyone was gone and fell in the pool.

There was alcohol at the party, and that's a factor in the lawsuit.

The man's parents had sued the trust that owned the house, but Demi had not been named. She's just been joined as a defendant.

As for why Demi's sued, the parents say there were no depth markers or any safety signage in the 10-foot-deep pool. The parents also claim there were rocks around the pool which created a trip hazard. And, the parents say, the temp in the pool was 101 degrees, and they say that compromises brain function.