Blender 2.8x also gives the option of making a Collection Instance. Any collection can be called up as a Collection Instance: ShiftA add > Collection Instance > the collection of interest.

A Collection Instance shows up as an orange folder in the Outliner, with the collection it was created from as its 'Data'.

A Collection Instance is transformed as one object, and gives no direct access to its components.. they must be edited in the source 'Data' collection. Collection Instances can be nested. As with Object instances, alterations made to the source collection are reflected in all the instances, and they are cheap, computationally.

You can append collections from an existing Blender project to another/new project by using File > Append. Select the .blend file that contains the collection that you want to append and open the Collection directory inside the file. Select the collections and click Append from Library.