I have visions of the OP sitting in Wimpy tonight with her friend and some very pissed off colleagues 
No Wimpys for us last night! We had a lovely night, and my friend didn't randomly show up which was a relief!
It was the week after next she was suggesting.
Wow, there's been a lot of responses since my last post, wasn't expecting the thread to be this popular! Thanks for all of the advice everyone. I'll try to answer a few questions to clear things up....
Elena - can I ask if any of your friends have accepted her friend request on FB? If they haven't, and might be considering it, I think I would do as other posters have suggested, and warn them what she's up to.
She actually added the 2 colleagues last Friday the day after we went out, they accepted out of politeness, I knew this at the time. But I found out yesterday over dinner that she'd messaged one of them that day (not the one she pouted at). This girl works on the floor below me (It's quite a big company) so I don't see her as much as the others. I asked if she wouldn't mind showing me the message- it was the same one she sent me on Wednesday about having a good time and wanting to do it again the week after next. The weird thing is it was completely identical, same grammer, same capital letters where there shouldn't have been, so she must have copied that message that she sent to my colleague on the Friday, and pasted it to me on the Wednesday. My colleague responded with a very brief "Yes nice to meet you, I'm sure Elena will let you know the details" because she didn't know what else to say! My friend then asked colleague how she was, but she ignored it.
I can't help but think there is something missing in this.. I was taking it all in until she asked to go again... thats when I thought something was not quite right. How close a friend is she? and if she was as unfamiliar with your town as needing a tour of the sights, why will she be there next week? and how did she know what you did every Thursday? How close a friend is this? She sounds vile, and an attention seeker, and I can only assume that, like a lot of attention seekers, she thinks her histrionics are amusing to others, and she wants another chance to perform. Seriously, I wouldn't give this nightmare the time of day. Yes, you should be embarrassed, not just by her, but by your insane feebleness in allowing her into your life at all.
How close a friend is she- close, known her since secondary school
If she's unfamiliar and needs a tour of the city, why will she be there next week- She won't unless she'll be out with us, that's the only reason she'd be coming
(getting weirder isn't it?)
How does she know about your plans every Thursday- We speak regularly on Facebook, and I post pictures.
UPDATE So for those who didn't see my update, here's the last part of it...
Friend: I was quiet because you were all talking about holidays and travelling and I haven't been abroad since school. Then you started talking about Donald Trump who I know nothing about. Maybe your friends could have made more of an effort to get to know me and talk about something I'm interested in?
I didn't respond to that as it was getting late and I was fed up with the whole thing, so I went to bed. I woke up this morning and saw she'd sent another message last night an hour later, saying "Even though I feel like I have the right to be angry, I've decided I won't be because it seems too petty to argue about. I think your friends could have made more of an effort, and the coffee thing seems like nit picking. You couldn't tell me what was so wrong with getting a coffee from next door, just that it wasn't the done thing. To me that doesn't seem like a good enough reason for Kate to talk to me like that and to not invite me. This is all making me feel like I'm in Mean Girls where I can't sit with you."
After this I lost patience, and replied:
"Okay I feel like you are backing me into a corner here. I've tried my best not to make things awkward but you everything I'm saying is just going through you, and it's draining the energy out of me. You joined our group and I thought we were having what would be considered a normal conversation over dinner, you don't have to be a traveler to contribute to the conversation. As for the Donald Trump thing, he's pretty mainstream right now. If you don't know much about him, why didn't you ask? Also looking back now, I realise my colleagues didn't mention work as much as they would usually, so to me that looks like they were making some conscious effort not to exclude you.
I'm not going to get into the coffee incident any further, to be honest it wasn't just about the coffee. You were bringing the mood down a bit by complaining about the food, when you knew we were doing to that restaurant. Also making a fuss over 70p didn't help, couldn't that have gone towards the tip? That was a rhetorical question by the way, I can't be bothered getting into this anymore. Like I said, I don't want to fall out with you and don't want things to be awkward. I'm leaving the ball in your court and hoping we'll laugh about this some time soon."
Well, I don't think we will. She replied:
"70p might not seem like a lot of money to you but it is to me, I don't earn a lot like you. And I didn't find it embarrassing at all, I'm not required to pay a penny more than I have to. Why would I tip when I didn't enjoy my meal? It was already expensive for what it was. And I don't have to get involved in a conversation when I don't want to. I don't really care about what Donald Trump has to say, what has he got to do with us?"
I don't earn "a lot of money", and she doesn't even know my salary! Speaking of which, in all fairness to her, she did open up a bit at one point... She told them apart her part time job and hobby, and that she only works X amount hours per week so she can still receive jobseekers allowance and afford to put time into her art 
Do you even want to be friends with her anymore?
Honestly I have no idea now, she's really annoyed me. I'm just not going to get through to her am I?