The best way to contribute is by continuing to use Compose, and submitting issues when you find them, and feature requests when it feels like something's missing.
As for code contributions, that's a bit of a complicated story with Compose, because of the split between Jetpack Compose (Google's androidx version) and Compose Multiplatform (JetBrains' fork).
Trying to contribute to Jetpack Compose is not straightforward, as contributions through GitHub are only allowed for the Compose Runtime/Compiler (not for UI). For me this was already large enough of a throw off to not pursue this much more, but you look up how Google handles contributions to androidx. It's a bit easier with JetBrains' CMP, but still not ideal.
And of course, contributing back to the third-party libraries you use is a great way to grow the Compose Multiplatform ecosystem into a strong and mature one. Most of them are on GitHub/GitLab and contributions are as easy as opening a PR (after discussing the issue/feature request with a maintainer in an issue first ofc).