Community
This is an open source project, and quite a lot of the help it needs isn't code.
Read the protocol
Perhaps the most valuable thing anyone can do is read the specification and tell us where it is wrong, unclear, or leaves something unsaid. A design document only its author has read is a design that has only been checked once.
Write an implementation
The specification is deliberately complete enough to build against, and there are published test cases to check your work with. If a third implementation disagrees with ours, that is about the most useful bug report there is.
Report a bug
Ordinary bugs belong in the issue tracker on the repository.
Report a vulnerability
Anything affecting confidentiality or authentication should not go in a public issue. See the security page, or go straight to security.txt.
Run a server
Every independently run server makes self-hosting more real, and tends to surface the assumptions we did not realise we had baked in.