I don't know of a tutorial, but I did find this package for Revolution:
http://modx.com/extras/package/importusersx
I've never used it, but if it doesn't do what you want, the code might get you started.
There is also this bit of code I wrote for someone a while back:
https://forums.modx.com/?action=thread&thread=72179.
It only writes usernames and passwords and puts the user in a usergroup. If you need to set user details beyond username and password, it gets a little more complex because they are in a different table.
For example, to set the fullname and email fields, you'd do something like this(after creating the $user $object):
<?php
$profile = $modx->newObject('modUserProfile');
$profile->set('fullname', $fullName);
$profile->set('email', $email);
$user->addOne($profile);
$user->save();
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html
