If you're interested in contributing to MoarVM, you'll likely want to work against the bleading edge. Find it on GitHub.
Join the #moarvm channel on irc.freenode.org. There, you'll often find various folks willing to help you on your way.
What do you enjoy working on? With most of the MoarVM team also being Perl 6 developers, we very much follow the -Ofun approach. For some of us, that's stress-testing various aspects of the VM (such as the GC), and reporting - or fixing - what comes up. For some, it's working on missing features. For some, it's profiling for hot spots and seeing what optimizations might apply. For some, it's doing the work to get MoarVM up and running on a platform it doesn't yet support. Find the itch you care to scratch, and join in!
So far, the community around MoarVM has been polite, professional, and light-hearted. A little patience, tolerance and forgiveness go a long way. Please help keep things pleasant.
As for the practicalities: