The Scions are comprised on the Circle of Knowing and the Path of the Twelve. The Path of the Twelve were originally a group of individuals with the Echo run by Minfillia, and when Louisoix died the Circle of Knowing joined the Path of the Twelve to add their own assets to them thus making the Scions. Possessing the Echo is no longer a prerequisite or majority of the group, but it's still desired in the Scion's agents (hence Arenvald)

I was redoing ARR's questline over the last few months and I noticed there are some people who swear they used to be in the Scions but don't remember their history so they may be other Echo touched individuals saved by Louisoix but that's just guesswork.