Atom.io: Emmet and jsx

Open Atom -> Preferences -> Packages -> Emmet

Scroll down a bit and you'll see a note about this particular issue. From there you just need to grab the correct context, which in my case was source js jsx and add it to your Keymap configuration.

 # Auto expanding for emmet @
 'atom-text-editor[data-grammar="source js jsx"]':
     'tab': 'emmet:expand-abbreviation-with-tab'

OR (with a more relaxed selector)

# Auto expanding for emmet @
'atom-text-editor[data-grammar*="js"].not:[mini]':
  'tab': 'emmet:expand-abbreviation-with-tab'

As of the latest version you can hit cmd + shift + e. I assume for windows or linux it would be ctrl + shift + e (Although I have not verified windows and linux one)