"George and Amal have great chemistry," a source tells PEOPLE. "Every time they mingled separately and locked eyes, George had a cheeky grin. You can just tell that Amal makes him very happy."
The couple, who arrived together in a chauffeured car, walked into the party holding hands and stayed close to one another throughout the celebration.
"They mingled separately several times, but it never lasted for long and they seemed more comfortable being close and hand-in-hand," says the source. "They stayed at the party for hours."
Alamuddin, 36, "seemed to get along great with all of George's friends," adds the source. The lawyer was spotted chatting with Cindy Crawford, who is married to Clooney's good friend and business partner, Rande Gerber.
U2's Bono also attended the event, and got the karaoke going.
"Everyone wanted George to sing, but he declined and said, 'No way' since Bono was there," says the source. "Bono instead entertained guests with an Oasis song."