Log in a user programmatically in MODx, answer (hopefully) to a question posted in the forums here; http://forums.modx.com/thread/86714/is-it-possible-to-login-a-user-programmatically-in-frontend

modx-programmatic-user-login.php

<?php

$id = 1; // just an example ... use appropriate id

$user = $this -> modx -> getObject('modUser', $id);

if (!$user -> hasSessionContext('web')) {
            
    $user -> addSessionContext('web');
}

?>

For more flexibility I use:

$contexts = !empty($scriptProperties['authenticateContexts']) ? $scriptProperties['authenticateContexts'] : $modx->context->get('key');
$contexts = explode(',',$contexts);
foreach ($contexts as $ctx) {
    if (!$user->hasSessionContext($ctx)) {
        $user->addSessionContext($ctx);
    }
}