One of the most used export formats in DOORS is the Spreadsheet format (or direct export to Microsoft Excel). This is also the only easily available file format which allows for offline editing as the corresponding Spreadsheet import (from Comma or Tabulator Separated Values files, CSV/TSV) can update existing DOORS data. These exports can also be imported to cloud-based Google Docs allowing for collaborating on DOORS requirements
If it is planned to re-import the modified data to DOORS it is useful to prepare an export view in DOORS. This export view mainly breaks up the main column to separate columns for Object Heading and Object Text so that individual attributes can be updated on re-import. The view should also include the Object Identifier column as this is needed during import.
After exporting from DOORS (File / Export / Microsoft / Excel (and then maybe save as CSV from Excel) or File / Export / Spreadsheet to CSV) the resulting CSV file can be imported to a Google Spreadsheet.
As see from the preview window, Google import for CSV file does not read the column names correctly from the first line of the CSV file. For this reason it is more useful to do the import to Google Docs straight from an Excel file in XLS / XLSX format, as this file seems to be formatted correctly: 
After import the file can be formatted in Google as needed: in the above example I have frozen the first row, set word wrap on for Object Text column and displayed in bold the Object Heading column and column titles in first row.
As the Google Docs is a cloud based service this file can be now published to specific users. The shared file can also have user settings: how can read the file, who can comment, who can edit the data.
The commenting option in Google Docs can be useful within Google while discussing the requirements, but these comments cannot be exported or printed. 
Modifications done in Google Docs are stored in the Revision history, allowing also to roll-back changes, if needed.
New requirements (new objects) can be added in Google by inserting new rows to the spreadsheet. No Object Identifier details are needed for these as DOORS will give the identifiers on import.
After modifications the file can be exported from Google to DOORS by selecting File / Download as / Comma Separated Values (DOORS can update data only from CSV / TSV files).
The generated CSV file can be imported to DOORS in the original module by selecting File / Import / Spreadsheet. The updated objects can be identified by DOORS by using the Object Identifier as unique key.
After import DOORS will display the result for the import: how many object updated and how many created. If there is a main column in the current view, the modified or created objects are shown as modified by a red change bar before saving the changes.
If your modifications or additions include characters outside of the normal ASCII symbol set (e.g. Finnish text) then it might be useful before importing check the import preview by pressing the Advanced button in the Spreadsheet import dialog and selecting correct “Encoding” for the characters – I used UTF-8 to import the ÄÖ-characters correctly.
For more information on Google Docs see Wikipedia article https://en.wikipedia.org/wiki/Google_Docs (also good to read the “Data safety and privacy” chapter here) or Google Drive help https://support.google.com/drive/?hl=en.
DOORS & RE blog 
Thank you for this page! One additional point that could help… While importing back to DOORS, I had to click on “Advanced” button, and select the “65001 (UTF-8)” encoding option because of accented characters.