DDD implementation

I'm new to Domain Driven Design and I have some doubt about some concepts (hoping that this is the right place to ask this).
I know that in DDD I should avoid an anemic model, so thinking about a social network model who should make (save) the friendship between two friends?
I imagine the situation like having a class representing the Users (using a Java-like syntax):

class User{
    String username
    List<User> friends
}

So, should it have a method to add a friend?

class User{
    void friendship(User friend)
}

or should I have to use a service to do it?

class UserService{
     void friendship(User user, User user2)
}