Credit: Agent 47/WENN

Heading for The Hills? Don’t forget your hat — that was the message for guests at Lauren Conrad‘s 22nd birthday this week.

Conrad celebrated with a predominantly female party posse at Hollywood’s Katsuya. But it wasn’t all play for the reality TV star.

Crew members arrived almost an hour before Conrad’s 9:45 p.m. appearance at the venue to smooth out technical issues and set up shots — and not the kind served in small glasses.

When the guests arrived — including Lauren’s Hills pals Lauren ‘Lo’ Bosworth and Whitney Port — they were all wearing party hats, and kept them on as they nibbled on edamame, spicy tuna and birthday cake.

“The group toasted Conrad’s birthday during their meal and couldn’t have looked happier,” one eye-witness tells PEOPLE. Conrad, who sipped a watermelon-cucumber mojito “was chatting with all of her friends and smiling.”

After cake from Baskin Robbins was served, the party dispersed and The Hills gals worked some set-ups with the crew for the show’s new season. — Abby Stern and Caris Davis Agent 47/WENN