I text invite a girl out for a drink.

She replies, 'who else is going?'

How would you interpret this?

if this is a girl you don't know before she may feel a bit uncomfortable...
so she wants some backups
invite a guy-girl friends of yours that are fun to hang around with, and than do your stuff with the desired girl while on the foursome date

that's an insurance for her.
unfortunately, things don't always go well between a guy and a girl.
just to make sure there won't be awkward moments, and just to make sure she won't be wasting time in case things get fucked up, she'd feel more comfortable with few extra people.
it'd be nice to have a male friend and a female friend to come along. if things go well, it'd be good for you to show her that you have cool friends which is a value.
and if things go well, you can ask your friends to leave a bit earlier when she's not around so that you can continue with the target which shouldn't be a problem if she has a lot of fun with you.

but always make sure that the female friend is someone who wouldn't talk shit about you in the bathroom in case they go together.

Here's what to say.. 'I was just thinking as a casual date to get to know each other, so didn't have in plans to invite the other but what do you think?' anyways my honest truth is that she might not be into you otherwise she wouldn't have said that but you just gotta wait and see

Tell her who all else is going. What she was thinking doesn't matter, you can't interpret it and it shouldn't change your behavior.