Postgres project is always looking for contributors. The project has a very nice, detailed, well-defined documentation on how to contribute [1]. The community is very welcoming, and has a process (CommitFest[2]) in place to ensure all submissions get their due attention.
FWIW, I am starting to make an effort (e.g. [3]) towards helping the newcomers get their work in shape to be acceptable by the committers.
Note: I'm biased towards Postgres community, since I've worked with them for many years. So others' opinion _may_ differ, but it's highly unlikely.