Guitarist Scott Kohler has claimed that the 25-year-old singer invited him and ten other strangers for the party - two days after she lost visiting rights to her kids.
The 29-year-old man said that Spears downed vodka, then stripped to her underwear and said: "Let's get in the Jacuzzi."
Kohler alleges the singer accepted an offer of cocaine from a fellow partygoer, after he "joked he wanted to do a line off her chest, and she agreed."
He also claimed that she "seemed drunk" and took coke herself. But she still passed a court-ordered drug test the next day.
"She was in a great mood. She didn't have a care in the world," The Sun quoted Kohler, as telling In Touch magazine.