Via the API add content to a cell, but not overwrite contents

There is not a way to append/prepend data, you have to get the existing data, manipulate it as necessary, and write the new final value.

Even with people columns you have to read back the existing IDs and write the new array of all IDs. See this thread for a feature request I made to add the ability to add/remove values from a set to avoid race conditions. New mutations: add/remove_to/from_column_value