This worked for me for having the file open in multi user by default:

1. Click the File menu.
2. Select Open or Restore Company.
3. Choose Open a company file, and then click Next. 
4. In the Open a Company window, check the Open file in multi-user mode box at the bottom. 

Each time you open the company file from the computer, the multiuser mode will be enabled. You can do this on every workstation.