First off, congratulations!
Secondly, I would double check with your insurance first before you ask for another test. Most insurance will only cover one ultrasound unless it's deemed medically necessary. If you can afford another, and another won't be covered, I say go for it!
67% isn't a very good percentage in my opinion either, especially if they are telling you it's a girl. More "girl" predictions are wrong than "boy" predictions.
My husband and I did not find out what we were having the first time around- mainly because we wanted to keep it a surprise, but I actually think it turned it out better to have several neutral items, greens and yellows, because I figured if we had more children and their gender wasn't what we had the first time, we'd already have a lot of stuff. When we had our daughter, we ended up getting a lot of girl things anyway.
Who is invited? That's up to *you*. When choosing my guest list, I thought about how much room we would have, how often I spoke to/how close I was to the guests, etc. All my aunts & female cousins were invited in addition to about 6 close friends, my mother-in-law and sister-in-law.
I hope you have a great time!