It should be pretty straight forward but for some reason I don't see this working. Once you upload an excel spreadsheet to a community I can open it inside of Connections and it highlights the cell I am selecting in blue. Now I would imagine that I can edit those cells and save the file (maybe as new version).
Taking everything a step further would be an implementation like google spreadsheets, where you can open a shared file and edit it, in parallel with other people. The cells selected by other people would be highlighted in a different color and their updates would appear simultaneously in real-time.