This is too much...
Remember that it is a girl you are texting, and you have good relations with her already... Girls take everything you say and double if not triple the meaning. If you say just that you are gonna go to another party, one of the first thoughts that will cross her mind is that you are gonna see other girls. So you dont need to stress it.
If she is into you, she will invent things to be jealous of.
I would say something like:
"Hey girlygirl, I got invited to another party that doesnt sound as much like a total cockfest as the one we talked about. Not sure what Im gonna do yet but it kinda felt like they wanted some guys to come to that party to even the numbers... You could dress up like a butchy mechanic and come along...

"
Dont forget the smiley...
Then when she inquires further, you may wanna go into details and tease a bit more... But you need to send something like this before bringing out the heavy guns, this is called calibration...