Decorator Pattern in PHP Simple implementation
Just because a class like User has a dependency on another class (like an objects instance of type NotifierService) it does not mean that class has to implement that same interface.  Here it seems that User should not implement this interface at all. You can remove the Notify() method on this class and just call the Notify() method on the passed dependency in your adduser() method.
I don't think decorator pattern makes sense at all for what you are trying to do here.  You are not really trying to "decorate" objects adhering to NotifierService interface with additional functionality related to that interface, but rather simply consume a class of this interface in User.  This is a straight dependency injection situation.
The class implementation of EmailNotifierService is pretty trivial here with hard-coded to/from/message values, almost to the point that this code does not meet code review criteria.  Surely this is not something that is production ready.
Don't echo out output from classes such as this. Leave that to some other functionality.  If you simply want to "log" out class activity, then actually log it (you know to application/server logs).
Why does a class called User deal with multiple users?  Should this class be renamed?
Don't start method/function names with uppercase letters like you do here with Notify() and SendMail().  This should be notify() and sendMail().
$notifierservice should probably be $notifierService (camel-cased). adduser() should be addUser().
You should consider adding validation on public methods, either through type hinting of parameters (like you do for the notifier dependency) or through guarding clauses at beginning of method.  Here, for example, you do nothing to ensure you are getting a valid user passed to your adduser() method.