Yahoo! Groups have for a long time had big problems with allowing people to create obvious spam groups (randomly generated names, no meaningful description and so on) and then add people without any confirmation. This was annoying but at least they have an obvious abuse reporting link in the subscription mails. Sadly rather than improve the situation they appear to have just changed the link to not be a direct link to the reporting form but instead to be a generic help page. The link didn’t change, the content just got even less useful.
I’m strongly tempted to just block all of Yahoo! from the mail systems I control.