Hi... I want to install external authentication module. What is the correct "_config.php" file I can use?
I've created a new file (empty) but the module doesn't run.. I've renamed the examples files as _congig.php but I get an error... What can I do?

Thank you..

Have you read this:

http://doc.silverstripe.com/doku.php?id=modules:external-authentication

What are you trying to authenticate against? You can't use a blank _config.php the documentation explains how to re-use one of the example configs provided in the download.

When I use one of the examples I get this error: “Fatal error: Declaration of ExternalAuthenticator::authenticate() must be compatible with that of Authenticator::authenticate() in C:\wamp\www\silverstripe-v2.2.2\silverstripe-v2.2.2\auth_external\code\ExternalAuthenticator.php on line 458”.

Do you know how can I resolve this problem?

I can't help unless I know what external source you want to authenticate against.

I want to authenticate against other SilverStripe users....
Do you know how can I do?

Unless you are chaining together that login to SilverStripe with another external source, it's pretty pointless using external authentication just to check SilverStripe users - as there already exists a perfectly adequate login for SilverStripe. The point of the SilverStripe driver for auth_external is to allow daisy chaining logins with other drivers - and wasn't meant to be used for individual login. Either way, if you enable the SilverStripe driver, you should be OK. You can achieve this by copying the example information from the docs/sstripe.php to _config.php and changing the details inside to match your installation.