[Baertram]

You do not seem to understand the client and server problem here:
How should other players see this marker then? You cannot just add a marker somewhere and ALL will see it.
It's not server saved. Its locally shown on your client side, for yourself only!

Groups are the only possibility to transfer that information, if you are using the same addon. The addon will save the marker locally for you, then transfer the data to the group via the server. Either via a map ping (not intened to be used for that originally...) or by getting the group members position by API functions directly (only working within the same group as server info is requested and used) and adding the marker at the positon then. This way all group members using the addon would get a marker at client side. This needs to be repeated on each position change of a group member.
So basically this also is stressing the server and the client. Maybe taken into account by ZOs, this could happen, or not and slow down the server if 100000 users use such addons.

But again: The API won't provide that info if you are in 2 different groups. And even a map ping won't work if in 2 different groups.
So you need to have something "server saved" to transfer the data, where different users got access to (even if not grouped).
And so far I can only imagine this could be the guild note, and would only work for users in the same guild then.

My conclusion:
As long as we cannot see the code of that addon where it seems to work I still cannot imagine this works across 2 different groups WITHOUT using external helper tools, or a "bug". At least it is not intended by ZOs and maybe be against their TOS then.