I've been on a few dates with this girl, but they have all been sober dates. We have mutual friends and are in a SPAM group together.
Anyways, I texted her asking if she wanted to get drinks tomorrow. Her reply was:
Yeah I'd be up for it you should post it in the SPAM group.

what is the best way to reply to this?
I was thinking of saying: if I wanted everyone to come I would have posted it there in the first place.
But Im not sure if this is the best approach.

Any advice?

Her "group" will always be around for the next outing, right?
I don't think it's a great sign if you've already been on several dates and she wants to include other people now.

What I did yesterday in a similar situation seemed appropriate. I was calling this girl I number-closed 3 days ago to take her out, she is Chinese which means a higher chance of being conservative and me having to walk on eggshells to make sure she's feeling comfortable. She mentioned her roommates in our first chat, so to make sure she wasn't thinking of bringing them, I just let her know that it'd just be me and her, alone in the crowd and out to have a little fun. I'm kinda dense when it comes to noticing IOIs, but once I made my point, her pitch got a little higher and she got more giggly for no reason.

Some of her defenses were still up and she wasn't sure if we should go to a bar or a nice restaurant, so again I made this to be about her. Since I was new in town, I let her choose the place -in fact, I did this with both girls I'm seeing this week- and told her that it didn't matter too much as long as she was comfortable with where we're going.

cliff:
- Setting the scene and wrapping it around her favorite subject (herself) gave her a clear picture and less nonsense to worry about. And it worked.