I manage IT for a NPO, and Concrete5 was available from our hosting provider as something that could provide Forums/Discussion Board capability.
After installing it, it's unclear to me how I can set up a discussion board. Can someone point me to docs that can walk me through this?
But if your needs are humble, consider concrete5's 'Conversation' block:
To add to what Gondwana said, that conversation block coupled with C5 powerful permissions capabilities and pagination can allow you to easily have a simple forum