How i have to set up Roundcube that only user that are in the postfix users table can use Roundcubewebmail?

Roundcube, Postfix Courier and Mysql are running on the same host but Postfix and Roundcube are using different tables.

I'm thankful 4 every hint.

You can't, because Roundcube doesn't authenticate users, your IMAP server does. You need to configure your IMAP server to do that.

But users Can login into Roundcube and I want it that way that only users that are regristred on my Server can use my Roundcube installation.

I still don't know what you mean. Only users with valid IMAP authentication credentials can log in to Roundcube. Anybody else will be refused access. What else do you want to accomplish?