has 'name' => ( is => 'rw', isa => 'Str', required => 1 );

has 'members' => (
    is  => 'rw',
    isa => 'HashRef',
    default => sub{ {} },
);

sub add_members {
    my ($self, @members) = @_;  
    $self->members()->{$_}++ for (@members); # no duplicates in list
}

sub list_members {
    my $self = shift;
    return keys %{$self->members()};
}