I've seen people ask about adding a forum feature before, and the answers have always centered around using Confluence with an add-on. From what I can tell, the add-ons are focused on full-license Confluence users, or would require clients to sign up for an Atlassian account at a minimum, and there is no option for portal-only customers (non-Atlassian account) to be able to interact with a Confluence-based forum.
In evaluating switching from our existing customer-facing ticketing/knowledge base/community product, I've found answers or solutions to all of my other roadblocks, but this one is persistent.
It is somewhat of a hole in service-management / external customer features and is unfortunate I cannot see where Atlassian is working to address this need.