I haven't gone through this one myself, however, I suspect what you want to do would be covered in the Access Security Seminar.

It covers users and groups as well as locking down the database. Essentially, your office code would be similar to a group functionality - anyone with that code is like a member of that group.

You might also want to check out this TechHelp Series, and also the below comment that adds to the series.
User Level Security
Comment ID: 99269

My Security system has 3 tables  UserT, UserGroupT, and UserXGroupT.  I also have a security Sub in my Global module to give permissions based on user group to lock out any one not in the group.  You could add a OfficeCodeT table and add child link into the UserXGroupT  table.  Then add the office code to your security Sub and call it in your form load event.  If they are not given permissions they don't even see other controls on forms

David campbell, would you consider selling me those tables and code?