For Contributing and a master list go to this link: https://github.com/o3de/community/blob/main/CONTRIBUTING.md

For those that are looking to join specific SIG lists, you can find them here:
Build SIG: https://lists.o3de.org/g/sig-build
Content SIG: https://lists.o3de.org/g/sig-content
Core SIG: https://lists.o3de.org/g/sig-core
Docs SIG: https://lists.o3de.org/g/sig-docs-community
Networking SIG: https://lists.o3de.org/g/sig-network
Platform SIG: https://lists.o3de.org/g/sig-platform
Presentation SIG: https://lists.o3de.org/g/sig-presentation
Release SIG: https://lists.o3de.org/g/sig-release
Security SIG: https://lists.o3de.org/g/sig-security
Testing SIG: https://lists.o3de.org/g/sig-testing
UI/UX SIG: https://lists.o3de.org/g/sig-ui-ux