I think you would get a better response if you gave people more info. Are you paying for this ad, or wanting it free? What kind of ad? Banner, button, what? What is the message. You should tell people what it is that you want and what your budget is.
Are you paying for this ad, or wanting it free? Yes I will pay What kind of ad? Banner, button, what? Banner
I guess there aren't too many yahoo groups owners around here...you should visit some of the groups and e-mail the group owners
I have a group with 950+ members getting about 500 posts per month. I can give you banner placement on the group page and/or in the emails that go out (text ad is probably better for the emails). Just pm me.
That depends on the niche/category. It's good enough for me though. I'm getting half a million email signatures a month from a group that others are managing and I haven't spent any time on in two years, so it works for me.