Code for database user access

I have a database with a field I want to restrict access to. Each user has a
user ID that they use to log into the network. That ID was used for the
Security Wizard. Some of the users are permitted to see SSNs and others are
not. The old version of the databse has code that uses the network ID to
determine if the user can see this info after clicking on the "Show SSN"
button. I am unable to replicate this code.

Also, I'd like to have the user's ID automiaticall populate the "Entered By"
field whenever they enter a new record.

I have a table for all database users listing their name and user ID. They
all have permissions to use forms only with one granted admin rights as a
back-up for me.