Is there an advantage to being able to @ mention vs. just # linking, as Fibery already allows?
Both this request and the Support Adding Types to People App one just posted take that stance that A: “People” is an app in Fibery (in my view it’s not, at least not by any definition I can see that they use), and B: that the best solution to existing limitations is to expand the existing “People” type/app/whatever so that it can do what you want. I am not sure I agree with this.
I am uncertain of the best approach (and trust that the team will figure that out in time 
), but I think it’s worth taking a step back to more specifically define the elements that are wanted/needed here, not as they relate to any existing seemingly related implementation, and think about other ways to approach it that may be more congruent with how Fibery already works. So for example, as I mentioned above, there is a desire to “@ mention”, but what is the specific reason this is wanted? What capability do you hope to get from this possibility, and is there another, perhaps better way to handle it?
My personal feeling is that the existing user management functions should not be “People” and should not be represented as an “App”, though it is indeed at present. Rather than seeing this highly specialized (workspace access control) functionality broadened to work like a type, I would prefer to see user management in its own area, and then to be able to add “Workflows” to any type that would do the kinds of things you are wanting. E.g. one or more of the following: allow @tagging (with what benefits additional to # linking?), linking to actual user profiles/access controls, etc.
In other worlds you build your “People” type however you want, in whatever App you want, then just add the link to actual system users if desired. And if you do, the links are, like any link, optional. Some “People” entities will have “system users” linked, others will not, but the “person” entity will still be useful without it. When such a link exists, it would e.g. connect @s to Activity Stream of that user, etc.