I think groups won't work for the building, that's only to make editing easier.
The desk, etc.., are they model or level?
If they are level, maybe you can better make map entities of it.
(to make a map entity:
- copy your desk (or something else) in a new file
- build the new file with your desk as a map entity
- to import it in your level, click 'object --> load entity, and set the file type on map entity.)