SQL server Principal and Mirror logins and permissions Issue
See the article Transferring Logins to a Database Mirror by Robert Davis, where he writes:

There are two main issues with transferring SQL Logins. Foremost is the SID (security identifier) for the login. The SID must be transferred with the login or it won’t automatically map to the database user. Secondly, the password is not available in clear text, so you can’t just read it from the database.
This script addresses both of these issues. It creates the SQL login with the same SID and it uses the stored hash of the password to recreate the password using the HASHED property to indicate that we are supplying the password already hashed.

There is a link at the bottom to download the script dba_CopyLogins.sql. This will sync your logins between principal and mirror.