If they are not interested in the guy.
I have gone out with this girl a couple of times.
When I have asked girls out, they would say as friends, or they would ask who else is going.
i think there are plenty of girls who may possibly (depends how obvious you are) take being invited out as just a hanging-out-with-friends thing. i guess just straight up ask if they want to go out with you on a date, to avoid ambiguity. hope thats what you meant?
If you're close then whats the problem? I always meet up with one of my friends every few weeks and we'll go out for dinner or a few drinks or something, she's got a long term boyfriend and i'm not interested. It's the only chance we get to see each other.
I don't think it really matters whether you clarified if it was a date or not. When she's there, flirt with her a bit and build a physical connection. It will become quite clear whether she's interested.
I've gone out for dinner and drinks with guys who were just friends, but these are guys I've known for ages and they know that I've got a boyfriend so it's obvious that it's not a date.
If you're unsure, then you could always ask? Or refer to it in passing as a date and see if she reacts/objects.