The primary source of information is the Proxmox VE wiki. It combines the reference documentaion with user contributed content.
Proxmox VE itself is fully open source, so we always encourage our users to discuss and share their knowledge using the Community Support Forum. The forum is fully moderated by the Proxmox support team, and has a quite large user base around the whole world. Needless to say that such a large forum is a great place to get information.
This is a fast way to communicate via email with the Proxmox VE community
Mailing list for users: PVE User List
The primary communication channel for developers is:
Mailing list for developer: PVE development discussion
Proxmox Server Solutions Gmbh also offers a commercial support channel. Proxmox VE server subscriptions can be ordered online, see Proxmox VE Shop. For all details see Proxmox VE Subscription Service Plans. Please contact the Proxmox sales team for commercial support requests or volume discounts.
We also run a public a public bug tracker at https://bugzilla.proxmox.com. If you ever detect a bug, you can file an bug entry there. This makes it easy to track the bug status, and you will get notified as soon as the bug is fixed.