- A user can have dependent menu's for every company?
- Now DEM is embedded in one of the live companies, what has to be
copied to create a seperate DEM company?

Regarding your questions:
1.- The user´s DEM is company dependent. I mean if a user changes to another company, the DEM must be created in the other company in order to have the same structure. Unless the brg module has been shared between companys.
2.- You must need to do a dump from your live company. And in the other company you must import that dump.

Hope this helps.

You should copy your version from your live company to your new company.

You should use sessions tgbrg1235m000 and tgbrg1245m000 for this.

With the help of these sessions you can customise what you want to import and export from one company to another.