Diddy and Kim Porter have had a longtime on-again off-again romance – but the pair celebrated in style together when the music mogul threw an intimate birthday bash for her in West Hollywood Monday.

Nearly 30 guests, including Grey's Anatomy star Ellen Pompeo and husband Chris Ivery, co-star Sara Ramirez and reality TV's Lisa Gastineau, gathered at Murano Restaurant & Lounge to celebrate Porter's 38th birthday.

Porter – the mother of Diddy's nearly 2-year-old twin girls and 10-year-old son – arrived fashionably late to the restaurant, which was filled festive with red balloons and streamers. The night's soundtrack was a mix of old school 70s R&B prepared by Murano owner Sandy Sachs.

At one point in the evening, a guest tells PEOPLE, "The group serenaded Kim to Stevie Wonder's "Happy Birthday," and then they presented her with a cake."

While Diddy stuck to ginger ale at the party, the other guests sipped on a specialty lemon drop martini made with Ciroc vodka designed for the occasion. A four course tasting menu created by Executive Chef Luciano Sautto was served.

And the group had the whole place to themselves: Although the restaurant is usually closed on Monday, it was open just for the private party, which started at 8:30 p.m. and ended after midnight.