Would it be beneficial to the promotion of my website(s) to join Yahoo Groups? How would I use them to promote my sites? How about Yahoo Answers? What about starting my own group?
Don't try out these groups. Why don't you promote them via different forums on the internet instead. Don't you think that is a better idea?
Both Yahoo group and Yahoo! Answers have huge userbase, it remains good tools for sharing information and exposure.
Promoting in Yahoo Group may help you to expose your self. But careful. It may cause made a scam to yahoo group inbox by the members.
Depends how you use them. YA is great - post a related answer +link for more resources. There's a way to work yahoo groups. Basically you join relevant groups and start posting like a normal person. Qeustions/answers. All that jazz. Basically after maybe a week or daily posting you make a post something like "hey guys, I've been around for a long time and I gathered a top 10 questions thing, here's what I covered. It's long so it's on my website <link>". How you use that link is up to you. Could be a squeeze page or a login to to a membership site where they can get the info. But you MSUT give the info cause it's what they want!
Yahoo Answers is good exposure. However, they are not real keen on you submitting your website as a reference. Take that from someone who was banned.
How often were you submitting your website in Yahoo Answers? I answer all types of questions but when I answer a question that is in the same line of my website I will then leave a link to my website ONLY if needed.