While it's fresh in my mind I just want some feedback on how it went down tonight.

Basically after a positive opening we get to the dance floor and before long make out and grind for 15 mins or so.

Then I said something in her ear along the lines of "lets go to [new venue's name], is that cool or do you need to stay with your friends?" I suspect this wording is a bit sh1t so feedback about this welcome.

She says she has to stay with her friends...

We dance another 5 mins, she says she has to go toilet. Last words exchanged before she went in was some jokey comment I made about me not following her in, she laughs. Point is nothing is said about "meet you here in a few mins" or w/e

QUESTION 1: Is it a bad idea for her to come out 5 mins later and find me waiting right there for her?

So I decided the ideal is she emerges and finds me having fun talking to other people where we were last together. I attempt this and she doesn't emerge for about 5 mins. Convos die out. So I decide to be 'nearby' texting (?)

Now I spot her sort of a couple yards away talking to her friend side-on. The layout of the bar means we can assume she noticed me and decided to play it like this.

QUESTION 2: Is this just her trying to gain relative value by making me go to her or is it probably a sign I'm getting rejected?

Some general suggestions:
Like you mentioned, it's not good to ask her questions like "is that cool or do you need...."
Better to say, "Me and my friends are going to Razzles" and assume non-verbally that she's coming, or "I hear the party is hopping at Rain, let's go"
Also, if she leaves to go to the restroom while you're with her, either you've been with her a while and she has been drinking enough to need to piss, or it's her period, you made her wet, and she has to readjust her tampon. I would never be found waiting on her outside the restroom; if she likes you, she will surely find you in the club.
Good luck.

I'd say it depends on the situation. If you are with her on a date/arranged meet then you should make it clear you are waiting for her but still look like your having fun (so the idea of chatting with a nearby group was good).

If this was a first night meet, then open another group anywheres, have some fun, and make yourself visible (you should always be anyways). If she doesn't approach you within 10 minutes then I'd return and approach her/her group again. Crack a few jokes, make random chit chat, then continue where you left off. Ignore subtle rejections because half the time they're not even rejections

You should never ask the question like that just state it hey lets go to such n such place, may add some information about the place... for example lets go to mikes nightclub they have an incredible selection of music and great drink prices... girls can't resist a place when u advertise it right... now about the bathroom thing u should only wait if she makes a comment about doing something right after u get out the bathroom such as (I gotta go the restroom after that I will go with u such and such) or ill be right back after I go the bathroom... in most situation u still want to chat up a group but make sure it has some guys in it... because if u do it with girls she might right u off as a player... never look like your not having fun always make it seem like your the life of the party...