If you are looking to run your application in a multi user environment, you will need to secure your DB. Microsoft Access security is not for the faint hearted. Here is a link to get you started. It's a bit long, but well worth the effort.
My understanding was that from a technical perspective you don't necessarily need to secure the DB but that everyone using a database as multi-user must connect to a common .mdw file for record locking and concurrency control.
However, securing the database is a good idea because it then forces everyone to use the .mdw file to connect. Just make sure everyone uses the same .mdw file and doesn't take separate copies of it!