You can contribute to prplMesh in various ways: reporting bugs, improving documentation, improving the code. The complete guidelines for contributions to prpl's several projects can be found here.
Daily conversations take place on Slack in #the prplwrt and #prplwrt channels. You can ask for an invite to anyone currently involved, e.g. by sending a message to an active contributer on gitlab.
High-level developer documentation can be found on Confluence, which can be edited in-place. The documentation in confluence is available for prpl members, please refer to the general contribution guidelines for ways to participate in prpl.