Around line 652 in users.php replace (or add below) the line:
$display .= loginform();
with:
$display .= newuserform();
If you replace the first line with second you'll only have the newuserform and not hte loginform. If you place the second line below the first you'll have both the login form and the new user form. You may also want to change the wording in some of the language files to reflect the changes you made.
-Vinny