When a team registers in a league, the manager and any venue managers of all the venues that the league is associated to will receive an email containing the details of the registration.  Similarly, when an individual registers, venue managers of any venues at which the user indicated they would like to play will receive a registration details email.

The manager and venue managers are set via the "Venue" details screen. You can change who is the main manager of the venue there: Config Settings/Venues/"Details" next to the venue you are interested in.  You can also add and remove "other" venue managers there.

NOTE: If you remove a venue manager from the venue managers selection box not only will they cease to receive the registration emails but they will also be unable to view any of the information about that venue (including leagues, teams and users).