Kitso,

I tried joining using my existing Yahoo username, but this is what I got:

Verify Your Email Address Home Help

None of the email addresses in your Account Info are available for this group membership.
We can not search for memberships with inactive or unverified email addresses. To search for memberships with unverified email addresses, please verify the email addresses noted below. Verification ensures that you are the owner of the email address. If the email address you used for this membership is not listed below, you may add the email address.

Do you know what I'm doing wrong?

Edited to add: I tried verifying my email addy too but that doesn't seem to work either.