How do you know when a party is a hit? When Jessica Alba, Victoria Beckham and Anne Hathaway are partying with the 'Brothers and Sisters' cast (along with one mysterious inflatable sheep) until closing time.

Such was the case at Matthew Rhys' St. David's Day party on Saturday at LA's trendy Palihouse Hotel. Things were going so strong that 'The Office's' Rashida Jones brought friends such as Alba from her birthday party in another section of the hotel to join in late at night.

"It was pretty crowded especially when Rashida and Jessica merged their birthday party at midnight," one guest tells PopEater. "It never really slowed down."

Victoria Beckham (without David) and Anne Hathaway circulated around the party, which featured a public outing of newly-engaged Dave Annabel and fiance Odette Yustman. The two happily relaxed with Annabel's co-stars, including his ex Emily VanCamp. "They were all hanging out at a for a while," says the guest. "Everyone looked pretty relaxed and happy."