SMART on FHIR Discussion Group: https://groups.google.com/forum/#!forum/smart-on-fhir
FHIR Community chat (for FHIR questions that aren’t specifically about SMART): https://chat.fhir.org
Cerner FHIR Developers: https://groups.google.com/forum/#!forum/cerner-fhir-developers
HAPI FHIR: https://groups.google.com/forum/#!forum/hapi-fhir
Epic FHIR Support: Open@epic.com