For a complicated set of reasons that has to do with the way Yahoo’s registration system interacts with the Groups membership systems, users who create Yahoo! ID’s and choose the Ymail or Rocketmail domains (rather than yahoo.com) may have trouble joining groups (via the "Join This Group" button on group home pages) or starting groups. These users may get caught in an endless loop when asked to create a profile.
If anyone you know is caught in this loop, they can get out of it by first creating an "alias" in this way:
Go to: https://edit.yahoo.com/identity/manage
Click "Add Alias" and enter your desired Alias in the box provided and click "Save" directly to the right. If the Alias you want is available, it will then appear automatically as an Alias. If the Alias is already in use, you’ll receive a message that says "This alias is not available. Please try entering another."
(Note: The Profiles Blog has some useful tutorials on this if you want visual instructions).
Once they have successfully created an alias, the user should have no trouble either starting a group or joining one.