So I've asked this girl out who I met on one of my friends birthday party's a few months ago. We didn't really keep contact after because I was pursued someone else who was at the party.

Anyway,

So I've asked her out to go to a pub this Thursday, and she's agreed, but she's now saying she could invite some of our mutual friends along and make it into a mini reunion type thing, "If you want". I'm not sure what to say now, do I say yes, or do I come clean that I'd rather just be chatting to her? She could want to invite other people to make herself feel more comfortable?

There's nothing wrong with her inviting some other people even though others might think differently.
If this makes her feel more comfortable then go ahead and let her invite more people.
You can always isolate her from the group once you're together. Use the social extra's to your advantage and there's no way she'll say no to a one-on-one date the next time you want to meet up.

Just don't worry about it. I've had this occasion before aswell with wonderful outcomes.

Just play your game right.