For example, if Joe logs in to Cloud9 and changes code in six files, then logs out, how would I know what files he has changed?
Right now, the only way I can think of is finding which files were modified last. From there on, you can use the File History viewer to see what modifications were made to each file by your team member:
Within Terminal:
find $1 -type f -print0 | xargs -0 stat --format '%Y :%y %n' | sort -nr | cut -d: -f2- | sed '/\.\/\.c9\//d' | head
I've taken this script and modified it to ignore files within the .c9 folder from this answer: How to recursively find and list the latest modified files in a directory with subdirectories and times?
Hope this helps!