[jankassens]

There is a tag on some Github issues "good first bug" that should be a good start.

https://github.com/facebook/react/issues?q=is%3Aissue+is%3Ao...

I think a lot of the core folks also hang out on IRC at #reactjs on freenode.

[ConAntonakos]

I think they're on Discord now on the Reactiflux server.