The 'friends' circumstance is the Consensus Phase of the date. She wants to get an 'outside opinion' of you.
Not on the first few dates. Your job is to get *her* to like you more, not her friends, not yet. Her friends could be c#ckblockers. Happened to me last night.
I'm talking to this gal at a bar, just happened to approach her while she was getting a drink. While I'm talking to her, her GF comes up and starts asking me hard questions. I handled it fine, but who needs that crap right away? Having a nice little chat and a drink and then out of nowhere WHAM right in the ribs by her dyke friend.
Meet the friends down the line AFTER she's crazy about you, because THEN when her c*ckblocking GF gives you a hard time, SHE'LL defend you, instead of having her opinion of you melded by her GFs.
Edited to add: I can think of more reasons... conversely, what if one of her friends likes you and wants you for herself? Then she flirts with you and your date gets ticked off? Who needs that? Or what if the friend likes you, and then tries to sabotage your budding relationship by telling your date why you're not good enough? Or like what happened to me last night, some don't want to lose the company of their GF to you, maybe she's the only one out of the group that attracts the guys for them...We are dealing with a tricky, stealthy species here, you know...
I was going to just stick around and pretend like I'm having fun if she showed up with her friends.
Lordy, lordy, no. You'd have to be a better actor than she is to pull that off. She'll know you're acting. Besides, then you're really just being her puppet performing for her benefit all night.
And if you get shot down by just one other gal while you're there, that's it! Your date and her friends will be laughing their asses off and see you as a loser.
By leaving, and remaining cool about it, she'll see that you aren't fazed by her, you don't need her. She can't get to you. She'll also see that she can't manipulate you to hang around. That's great stuff right there. Other people are watching. There may be some other gals there that see this and think, "that guy's cool."
By leaving, you're free to go do whatever instead of performing for her all night pretending to have fun. Pick up other girls somewhere else, that's your compensation. Plus, she may wonder what you're doing if you don't hang around to obviously satisfy her curiosity about it.
Be the first guy she's ever encountered that doesn't bend to her whims. It's a new year, man. Time for a new start to things.