Lohan rolled over to Hilton's beach pad for a small shindig with some of Paris' close friends ... including her sister Nicky, actress Emma Roberts and "Inception" star Lukas Haas.
One source at the party tells us, "Everyone was so surprised when Lindsay walked in and we thought they didn’t get along. But they immediately gave each other a big hug."
The gang kicked it on the beach, in Paris' hot tub and later had a mini dance party ... but not once did anyone see Lindsay take a sip of alcohol.
We're told Paris is "very excited" about rekindling her friendship with Lohan ... adding, "She LOVED seeing her again."