Hmm
It seems like that nobody is able to migrate from MSN to worldgroups anymore.
It's strange. On there wellcome screen they say: No Credit Card Required. Moments later the want the card number?
They also say it's free but there "Terms and Conditions of Use" state that:
Free registration: If a credit-card was used to secure and validate a free registration, a temporary authorization amount of up to $1.99 USD may be applied to ensure accuracy and validation; the bank may charge additional transaction fees. In the event of any inconvenience, unintended transaction activity and/or surcharges imposed by your financial institution, click the "Contact" or "Customer Service" links at the bottom of this page to obtain an immediate courtesy refund. 
Nothing I can do to help from the inside of worldgroups, sorry.
I don't think it would be wise to give them any credit card info. I know I wouldn't.
Pilot_mol