updated 10/08/2013 AT 2:40 PM ET
•originally published 10/09/2013 AT 6:00 AM ET
Simpson, whose actual birthday is Oct. 3, Ross and a group of 10 guests gathered Saturday for a late dinner around 9:30 p.m at Buckhead hotspot Umi, where they enjoyed nigiri, sushi rolls, sashimi, rock shrimp tempura and miso-marinated black cod.
Ross is in Atlanta filming the final installment of the Hunger Games franchise, Mockingjay, and was joined by his costar Wes Chatham, Jovar Andrews (who was also celebrating his birthday) and DJ Rukus.
“Ashlee and Evan were really sweet. You’d definitely know they were dating if you saw them together,” an onlooker tells PEOPLE.
Around 11 p.m., Liam Hemsworth and three male friends arrived in time for cake (one vanilla and one chocolate) and cocktails.
“Liam was solo. Of course, all the girls in the restaurant wanted to sit close to him, but nothing. He didn’t seem interested. He just came with his boys,” the source adds.
– Jennifer Bradley Franklin
— People magazine (@peoplemag) August 7, 2012