Go-with person

Someone who's up for anything, and normally in a good mood to do it, regardless if they know what's actually going on.

"Me and Dan are gonna go and dress up like women, dance in the streets and pick up some studs, then surprise them with our penises... Hey, Ben, you wanna do it?"
"What?"
"Do you want to join us?"
"Okay!"
"You've always been a go-with person"