Three Firefox developers from three different teams (Paolo, Irakli and I) each came up with Promise (with slightly different name and semantics - I think that mine was called Future, for instance). We were using them behind the scenes to refactor Firefox and make its UX async. Of course, when we discovered that there were three implementations, we merged them.

Later, Irakli led conversations with (if my memory serves) two other teams who had also come up with Promise on the web or on Node, and we eventually merged with these proposals.

Similarly, my team was using async/await in 2014 or 2015 (it was called `Task` at the time), a few years before it was standardized. I believe that Paolo was the first person in the world to ship production code with async/await and me the second.

That was on a whiteboard. I could try and find the list of people who were in the room, if necessary :)