In similar form to what you did to get it installed the first time (Also its now possible to install without a mail server, but not yet to add users.)
When you attempt to add a new user as is, it generates and mails the user a registration key which they can use on the register link from the login page. So for a temporary workaround we are going to just send the registration key back to you (you can also view a list of active registration keys on the site admin page). So this will still ask for and use the email (to validate user to key).
In the actions/user-add.php find line 50 (at least it's 50 in the latest version) and change it to be this;
set_warning('There was a problem sending the email. The registration key is; ' . $user_key);
I believe this will send it back to you just fine. If this fails to work for you, simply delete lines 49 and 50. (this is the part that looks like this)
// send email
if(!mail($email, $subject, $body, $headers))
sendback('There was a problem sending the email.');
Let me know how this goes.