How to conference call with FaceTime audio?
It's not possible to have group call with Facetime Audio at this time. Apple used to have this feature in iChat but removed it because it was a poor experience. 

There are other third-party apps that have this type of feature and those might fit your needs. 

EDIT: Adding additional info from the discussion in comments.
The date of that article was June 2014 and was talking about pre-release iOS 8 (which came out in Sept 2014). Maybe it was available in the beta, I don't know. Like you, the articles I read all came out around the same time and it looks like they're all using the same wording leading me to believe they're all using the same source. I could be wrong but I can't find anything on Apple's product or support pages indicating how to do this.