and hubby Josh Duhamel
were in great spirits Thursday night when they made their way to West Hollywood hotspot Chateau Marmont for a pal's birthday.
The pair, joined by a small group of friends, were involved in "enthusiastic group conversation" throughout the evening, an onlooker tells PEOPLE. "Fergie wore a big smile for much of the night. It was obvious she was having a really good time. Though the couple's PDA was at a minimum, it was clear the two were enjoying the company and the conversation."
About an hour after the festivities kicked off, a small dessert with a candle was brought over to the table, and Fergie and Josh joined their pals in singing "Happy Birthday" to their friend.
"With additional desserts arriving, Fergie and Josh joined in on trying each," the source says, adding that the gathering came to an end at around 11:30 p.m. with Duhamel putting his arm around his wife as the duo headed to the valet.