If you want to help, do as I did. I started by installing and maintaining versions of coppermine on my own machine at home.
I have a copy of 1.2 running and installed, as well as a 1.3 from cvs, and at least 4 different devel installs from cvs.
I try to recreate the users problem then modify code to suit. When using one of my cvs installs when I'm done with that users request, I'll do a cvs update clean, and my galleries go back to default Coppermine code.
I would also suggest: pick a small area where you concentrate your effort. ie: pick an area you want to work with and research, and answer questions in that area. Learn that one area really well, then start branching out as you have time and interest. Instead of trying to answer every Coppermine question that gets posted as you'll get quickly overwhelmed.
Thats how I started, I began with the themeing engine and slowly worked my way around. There are still plenty of areas of Coppermine that I haven't explored, much less could answer a question on. But I know the theming engine inside and out and that helps me when I'm offering support.
If theres an area right now that I think needs more assistance is with file upload. I don't think this is necessarily a weakness in the coppermine code, but it has a lot to do with the variety of server setups and configs. If you were to pick an area to start with, helping folks work out thier upload problems would be a great help.