its not dofollow, yahoo is using nofollow Attribute in Groups. but it Can be Help to Increase more traffics for your site but not a PageRank Juice. Why not try with comments on Blogs ? Thanks John
Yahoo! is a party pooper. I just started my website about 4-5 months ago. I was a complete stranger when it came to nofollow/dofollow links. I posted about 350 best answers on Yahoo! Answers. While the traffic was great it didn't improve my page rank. I found out about 3 weeks ago that it was nofollow and I stopped posting. Oh well, I still get plenty of traffic daily from Yahoo! Answers.