1. Firstly, are you aware that Workgroup / User-level security only works with MDB and MDE format?
2. If your dabase is in MDB/MDE format, go to File tab / Users and Permissions / User-Level Security Wizard... and see if the Wizard allows to to join an existing WorkGroup. (I don't use ULS so I have only the option of creating a new workgroup enabled).
3. Failing that, I think you need to use VBA code to run the SetDefaultWorkGroupFile Method of the (Access)
Application object. This method is a hidden member so you need to set the "Show Hidden Members" in the Object Browser to see it. Once you see it, you can use the inbuilt Help button in the Object Browser to
go to the Help Topic on the SetDefaultWorkGroupFile Method.