Not the shortest, but see for yourself.

#!/usr/bin/perl -w
use strict;
use Set::Scalar;

chomp( my @l = <DATA> );
my %is_watched;
@is_watched{ @l } = ( 1 ) x @l;

my $prev = Set::Scalar->new();

{
    my $cur = Set::Scalar->new( grep $is_watched{ $_ }, split " ", `us
+ers` );
    my $t = localtime;

    print(
        map( "$t $_ has logged in\n", ( $cur - $prev )->members ),
        map( "$t $_ has logged out\n", ( $prev - $cur )->members ),
    );

    $prev = $cur;

    sleep 2;
    redo;
}
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