For a question abc, I would like to add multiple email accounts to it (notify abc of daily answers) so that some of friends can be alerted of possible updates to abc.
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This isn't possible -- otherwise you could "sign up" a bunch of people for notifications and effectively annoy them with spam.
At the very least the emails would have to be confirmed before this could happen, making this impractical.
I suggest you encourage your friends to subscribe to tags, via email or RSS, instead -- http://blog.stackoverflow.com/2010/12/subscribe-to-tags-via-emai/