There are a couple of ways to do this that are pretty straightforward. I create my own 'user' table in every app that i build which can be used for a myriad of things. However, if you want to just track this one 'user' role, create a table called 'settings' (or whatever you want). Add a user field called 'current team leader'. Create one record, and one record only for this table.
The record ID, if you only create one record, should be '1'. Create a relationship to this table from your tasks or milestones table. For the value 'related setting', set the default value to '1'.
From this point on, whenever you change the 'team leader' user in the 'settings' table, it will be reflected in all the other tables that you relate to 'settings'. You then set the email notifications to the email address of the 'related setting - team leader'.
