The NBA star was seen hanging out at the Marquee Nightclub in Sin City early Sunday morning (around 1:30 AM), where we're told he was with a posse of 3 ... one dude, who seemed to be his wingman for the night, and a couple of girls. One for each guy.

Eyewitnesses tell us the group was definitely together the whole time and seemed to be having a good time -- dancing with each other and chatting up with drinks in tow.

At one point, we're told the woman that Tristan appeared to be in lockstep with actually danced up on him ... and that they were very touchy-feely. As you can see in this video, obtained by TMZ, Tristan looked to be holding on to her tight, and whispering in her ear.

Eventually, we're told the crew left around 3 AM. Unclear who any of these people with Tristan might've been -- they're certainly not anyone we recognize in his immediate circle.

Now, we know what everyone must be thinking ... what about Khloe?!? Fact is, this isn't nearly as scandalous as it might appear ... as people well know, Khloe and Tristan are not romantically involved at the moment and they simply co-parent, which we've seen.