Zen Cart is actively seeking contributors to the 1.6.0 version of the product, which is called the "develop" branch in Github. There is a documentation page about Contributing to Zen Cart which contains all the information you need to get started.
Another option is to write a plugin for Zen Cart and contribute that. Here are the standards for Zen Cart plugins. When you have completed development, submit your plugin to Zen Cart, and it will be approved shortly! If you'd like to update an existing plugin, just go to the plugin's page, and click the "Submit an Updated Version" link at the upper right.
Yet another way to contribute code to the Zen Cart project is simply to answer programming questions on the forum.
Thanks for your interest in Zen Cart!