We're a very inclusive bunch here at Apollo, so anyone is welcome to contribute. Just fork the project, create a pull request and we'll work together to have it merged.
If you're unsure whether your pull request or feature is something that has a chance of being merged, just open an issue and ask away!
If you want to contribute to Apollo server, but aren't quite sure where to start, take a look at the roadmap and design docs. Just pick one of the upcoming features that you're interested in, and start working on it. If the design doc isn't clear enough (which it probably won't be), open an issue thread so we can discuss it.
Last but not least, make sure to join the Apollo Slack channel, where there are lots of other friendly contributors to talk to.