Inform user with account creation email, for existing users to Is there a way(It doesn't matter if passwords need to be recreated/generated) to send all the users an email about there account being added to the website after they are already added?
I'd be really interested in this idea too.  I'm making a website to contain and extend an existing registry and it would be great to set the whole thing up with all the members' details and then just send out a mass email containing each user's username and generated password when everything's put together...

I haven't tested it yet, but it could potentially be done:


Set up your users, ensuring 'Notify User' is unchecked and Status is 'Blocked'.  If you already have active users, block them (make sure no email goes out, check admin/config/people/accounts)
When you're ready to notify the users, go to admin/people and check all the blocked users, then user Bulk Options to Activate them (if there's a huge amount of users you might want to do a group at a time to save the server a bit...)
Individual emails should be sent out containing a 'one-time login', see admin/config/people/accounts and scroll down to E-mails and choose 'Account Activation' for the email which goes out.


I will give it a shot and let you know if it worked.