Saying you're down to do something means the same thing as saying you're up for it.
"Do you want to do this thing with me?"
"I'm down."
"I'm up for it."
"Do you want to do this thing with me?"
"I'm down."
"I'm up for it."
In my friend circle we will invite each other to stuff and ask "are you up or down?" Then schedule them regardless of response.