Ask HN: What software (if any?) should I use for a closed B2B discussion forum

We (http://timeblock.com) are building a SaaS app for our new working method TimeBlock.

To help our customers I an wondering if we should build and gove access to a discussion forum where they could ask questions, help each other and find motivation.

Would you recommend that we create such a site, and if yes - build on what platform?