Heading out for an approximate 40-person guys' weekend to celebrate the 40th birthday of producer pal Luke Watson, Kutcher and his clan kicked off the festivities with a viewing of the Clippers game in a private dining room at Stack restaurant inside the Mirage Las Vegas.
"Ashton was super nice to everyone and while he was there, he left for a minute to take a call from Mila [Kunis]," a source tells PEOPLE of the actor's pregnant fiancée, who he reunited with after the weekend away.
On Saturday, the crew was spotted at Liquid at Aria, arriving at around 1 p.m., according to the source, who says Kutcher "was super casual, wearing a Chicago Bears hat."
The group "had a few drinks and Ashton stayed for about two hours."
That night, they went to Haze nightclub and hung out on the second level, which overlooked the deejay booth and the whole club, the insider reveals.
"They arrived around 12:30 a.m. and were all wearing Dallas Cowboys hats," the source says, adding that they were "out and about until around 3 a.m."