User Guide
The User Guide (which is also included with ZAP) is a good place to start.
User Group
The User Group is the best place for questions about using ZAP.
Developer Group
The Developer Group is the best place for questions about working on the ZAP source code or extending it via add-ons.
Scripts Group
The Scripts Group is the best place for questions about extending ZAP using scripting.
IRC
For more interaction you can also join the Libera Chat #zaproxy channel which is where many of the ZAP core contributors hang out.