The Zotero toolbar can also be used in Google Docs, and makes therefore collaborative work easier. It is part of the Zotero browser connector. A toolbar and a Zotero button are added in Google docs, and can be used to add citations and a bibliography, the same way as in Word or LibreOffice.
Zotero recommends to avoid having several people insert or edit citations at the same time. When you are ready to download your document, in pdf or Word format for example, you must make a copy (menu file in Google Docs) and "unlink" the citations, to convert them in plain text.