Although Simple Forums works almost perfectly out of the box you’ll have to make one very minor modification to get users working (otherwise you’ll have site sql errors displayed).

The reason for this is that in MU wordpress the prefix is appended with the blog ID but the users table is global and not in every unique blog.

Anyway its a very simple edit.

open up sf-includes.php

and at line 18

change

define('SFUSERS', $table_prefix.'users');

to

define('SFUSERS', 'xx_users');

Where xx is your wordpress prefix (wp_ by default).

Everything should work after that.