splitting an Access into a fe/be has nothing to do with security of access to the data. it does have everythign to do with reliability fo access tot eh data.
if its a multi user environment then you should be using a split fe/be model
read up on the Access security FAQ. it may niot be releavtn to what you are using as modern versions of Access no longer sue the workgroup security. so make certain you read up ont eh hiws, whys and wherefores of securing an access application AND data
but if you do go down the route of splittign into a FE/BE and/or apply security alwasy make certain you have lots of backups at each stage, take copious notes of what you have done,. any passwords, group accounts and so on. before deleting any of your backups THROUGHLY prove your changes are right. that those who should have access still do. prove your backups are 'good' ie readable BEFORE relying on them
in the workgroup security model you can assign users to groups, permissions to groups (no remove permsissions from groups. thats how you control who can do what.
but there is one other key element that is often overlooked in Access Security and that is securing the data from the user. because of the nature of Access it can be tricky to fully secure your data. read the appropriate security FAQ
I'd rather be riding on the Tiger 800 or the Norton