The easiest way would be to join the workgroup in question, log in as an administrator, create your database, then rejoin the SYSTEM workgroup (the default workgroup).
1. First, join the workgroup you want to use to create the database. Go to Windows Explorer and navigate to C: > Program Files > Microsoft Office > Office > MS Access Workgroup Administrator. Open the shortcut and select "Join". Then navigate to the folder where the workgroup you want to use is located and select it.
2. Create a blank new database. After the initial save, you will be asked to log in. You will have to log in as a user with administer rights in the workgroup you've joined. Save the database after you've logged in and close it.
3. Rejoin the default Access workgroup. Open MS Access Workgroup Administrator again, select "Join" and navigate to C: > Program Files > Common Files > System > System.MDW. Rejoin that workgroup and exit the program.
4. Create a shortcut for your new database that references the workgroup (just make a copy of the shortcut I presume you're using for the current database and change the command line parameters) and login as the administrator. Then you can create your objects, assign permissions, etc, ad infinitum.
No Access has one security file in any one instance
You can however specify what workgroup file to use iwhen you open acsess via a shortcut, offhand I think its the /w switch. have a google for access command line switches.