The guestbook module has to be inserted like any other Profile element: something like this:

if (module_exists('guestbook')) {
if(!$user->guestbook_status){ // guestbook_status seems to store 0 for enabled and 1 for disabled
print module_invoke('guestbook', 'page', $user->uid);
}
}

The default guestbook doesn't work the way we're accustomed to: i.e. when i click on a user's avatar from a comment they left, it takes me to their guestbook page, not their profile page. This will require modifying the guestbook.module file.

-------------- update

So I pulled up my old rollsonline guestbook.module file and I recalled the modifications i did on that site:

In particular, The guestbook comment avatars had to be redirected to that indivudual's profile page, not their guestbook, so here was the modification to the function: theme_guestbook_user_picture()


if (variable_get('guestbook_mode', -1) == GUESTBOOK_SITE_GUESTBOOK
&& user_access('access user profiles')) {
$user_link = "user/$account->uid";
$user_text = t('View user profile.');
}
else if (user_access('access user guestbooks')) {
$user_link = "user/$account->uid"; /*MODIFED BY ROLLS to redirect users to profile page*/


-------------- update
removed the Link to USER's profile page that shows up above the message box. I felt it was redundant to have that link on there if you're already on the user's page.

In: function _guestbook_user_profile_link($uid), remove the line:

/* $output .= '' . t("Visit %name's profile", array('%name' => $namelink)) . ""; */
/*REMOVING PAGE LINK - ROLLS */