Be aware that Yahoo is now using "Web Beacons" to track every Yahoo Group user. It's similar to cookies, but allows Yahoo to record every website and every group you visit, even when you're not connected to Yahoo. Look at their updated privacy statement at
http://info.yahoo.com/privacy/us/yahoo/details.html
About half-way down the page, in the section on *cookies*, you will see a link that says *WEB BEACONS*.
Click on the phrase "Web Beacons." On the page that opens, on the left find a box entitled "Opt-Out."
In that section find "opt-out of interest-matched advertising" link that will let you "opt-out" of their snooping. Click it and then click the opt-out button on the next page.
Note that Yahoo's invasion of your privacy - and your ability to opt-out of it - is not user-specific. It is MACHINE specific. That means you will have to opt-out on every computer (and browser) you use.
Alternatively you can go to NAI (Network Advertising Initiative), and opt out of upto 25 different Web Beacon systems (including Yahoo and Google) at
http://networkadvertising.org/managing/opt_out.asp