At the moment, all site-admins of the instance will receive notification when a new user has self sign-up.

An email similar to this one is sent and a banner is displayed for all site-admins until all invites are either dismissed or accepted.

Suggestion

Add a switch in the User management section > Self-signup so site-admins can choose whether or not be notified upon user's sign-up.

Note: Clicking on the "X" will dismiss the banner for an hour.