It's usage is identical to the ReactCSSTransitionGroup, however it has an additional prop called transitionAppear which takes a boolean value. This prop will handle the conditional rendering based on mounted state, then simply wraps the elements you want to animate with the regular ReactCSSTransitionGroup addon component.
To use this component, simply include it in your app with the rest of your components and use it in place of the ReactCSSTransitionGroup add-on.
It seems like the ability to apply a transition when the component first appears should be available in the ReactCSSTransitionGroup component soon. There is a pull request on Facebook's React.js GitHub repo which implements the functionality and it was merged in to the master branch last November. It has yet to make it in to a stable release, but hopefully it's not too far away.