HangHub by CKSource

With HangHub you can view other users who are working on the same GitHub issue or pull request as you.

You will also be able to check whether other users are viewing, away, commenting, editing or merging in real time. It’s possible to disable the extension for specific organizations or repositories.

After you install HangHub, you will never have to worry about wasting time working on the exact same thing as your teammates at the same time.