John,
I went through quite a bit to do the same in a secure manner. I ended up installing Sudo and giving the webuser permission on it for certain commands.
Then it was a question of defining a sudo library file:
php_sudo.lib.php
$sudo_exec='/path/to/sudo';
$adduser='/path/to/adduser';
$other_commands...
The exec
exec("$sudo_exec $adduser ... ");
Do note: if you plan to give these options to your users, a lot of security and error checking has to be in place.
Let me know if I can assist with more details.
-m.