By personal experience, I would not suggest you to do it as every new post made in a yahoo group sends an email to all its members. This way if you post your link in any of the groups it will automatically email it to all the group members & might be considered as “email spamâ€.
I dont think this is spam or against TOS. You could be using Yahoo groups to promote your site, rather than sending your adsense code to the group members. If you send your adsense code in the emails, or tell the users to come and click on your ads, then its against TOS.
It really depends if it's on topic. But of course you can use yahoo-groups to promote your website. I use it for my newsletter. On a discussion group though it is impolite to overdo it. People will start responding negatively if you cross the line.