Uses include: "She's down," an absent member of a group is known to think something is a good idea or wants to do it and "I'm down with that," I like that idea, I want to do that, include me in, etc. I recall hearing this expression in 1991 in New York City but think it may be much earlier. I would welcome anyones thoughts on this one as I was unable to Google it.
The first time I heard "down" used in this upbeat way was in 1984, at the end of a free summer concert in Prospect Park, Brooklyn, by a member of a peppy funk band called Urban Verbs. One of the saxophone players said something like "Any time y'all want to do this again, we're down!"
But it turns out that "down" in the sense of "up" goes back at least to the mid-1940s. J. E. Lighter, The Random House Historical Dictionary of American Slang (1994) offers three relevant (and closely associated) definitions:
down adj. ...6. Black E. a. ready and eager for action; (also) formidable in a fight; tough.
1944 Burley Handbk. Jive 137: Down with It...to be ready for action. 1952 Ellson Golden Spike 15: Are you still down for it? Ibid. 69: Let's jump them now...Is everybody down? [Additional citations omitted.]
b. knowledgeable or conversant, esp. thoroughly.—constr. with with; (hence) smart; canny; sophisticated; HIP. [Cross reference omitted.]
1944 Burley Handbk. Jive 15: I'm with it....I'm down with the action to my own satisfaction. Do you dig? Ibid. 47: That square, Iago, is down with the action. Ibid. 137: Down with it—To understand, know. 1955 in R.S. Gold Jazz Talk 75: I don't know who the singer is, 'cause I'm not down with all the singers now. [Additional citations omitted.]
c. eliciting a strong emotional response; most enjoyable; excellent—also constr. with with it.
1946 Mezzrow & Wolfe Really Blues (gloss.): Down with it. Top notch, superlative. 1957 Simmons Corner Boy 209: She has the downest jukebox in town. 1964 N.Y. Times Mag. (Aug. 23) 1964: Down: Hip, just right and true, as in "That was a down movie." [Additional citations omitted.]
For further definitions and examples (given in response to a somewhat narrower question), see Hugo's answer to What's the origin of "I'm down with it"? (cited by Josh61 in a comment above).