I have a client who wants two people to get the emails when someone registers for the site. Will that happen automatically if I create  a second admin user? Or can I add an additional email to the main site admin?

In one scenario I have about 4 people maintaining and reviewing DNN user accounts.

There is one mailbox they all log into, and when they've dealt with the notification they delete it in that one centralised mailbox. That mailbox will send notification out to other email addresses if we really want to, but we find just logging into mailbox maybe once or twice a day is fine and personal email notifications are not required (just notifications for the admin account).

This avoids duplication of work (when somebody has already viewed and processed a notification), and stops people being over reacting (instantly pouncing on registrations)- when there really is lots of other work to do.... in reality we don't even need to look at the notifications if user accounts and other stuff are viewed regularly - but people do like to work that way I know.

If the user just needs a report of user registrations say in a week/month/day, there are modules out there which will do the job.