Right-click on the root node in the tree showing your message store. Select
Properties. General tab. Click "Advanced" button. Add/Change/Delete the
password on the .pst file. Everytime you load that .pst file, like when
opening Outlook if that is the default message store, you will be prompted
for a password to access its contents.
Rather than trying to share the same mail profile with multiple users, why
aren't you provided a separate Windows account for each user so they each
have their own separate mail profiles (and data folders, and tweaks, etc)?
