You can add articles to your shared library in a few ways:

1. By importing PDFs or export files from another reference manager directly from your desktop/local storage, either by drag-and-dropping them from local storage to the shared library in Papers, or by opening the shared library and clicking on the Add button, and then selecting the files for import:

You can import those files to specific folders within the shared library, just make sure you are positioned within that folder when importing those files by drag-and-drop or make sure to select the correct list if using the Add button: 

3. Add articles from your personal library by selecting one or more and dragging them to the desired shared library:

You will be prompted to select whether you want PDFs included, as well as annotations and duplicates merged or not. 

Once those references are in a shared library, all team members within it will have access to them.