William Stein ([email protected]) wrote most of CoCalc (formally called "SageMathCloud"). See the git history, which as of Oct 30, 2015, has 96% commits by him.
For a recent breakdown, run
git shortlog -sn --no-merges.
Everytime the GPL licensing note refers to the "Authors of SageMathCloud", the following (maybe incomplete) list is meant.
To extract the names of all Git contributors, run this piece of Python code inside the codebase: