Well if you know it is possible then ask these guys as we all know it is not if you ARE NOT GROUPED
Edit:
No, again: If you are not in the same group you got no access to these other groups/non grouped overland/whatever other units!
You cannot see where they are, cannot access their nameplates in teh world, cannot access anything about them via addons or you'd be able to mark hidden players with a marker icon e.g which would be cheating!
Whatever your other guild told you it's either no addon or they found a way ZOs does not want it to work, believe me.
But if you find a solution that is legit and ZOs allows it, feel free to tell us all please!
Example:
The unitTag for grouped players in the same group are group1, group2, group3 up to group12.
But you cannot read data of other groups, just your own one.
So no, you cannot use that functions to set an icon as you got no access to these other grouped players, not at all!
You could only redirect the textures of your own 12 group members.
You cannot even share data to other groups via MapPings, like combat sharing addons do, as players that are not grouped with yourself cannot read the mapping data of other groups.
And you are not even able to circumvent this by sending the data via guild notes to other guild members as changing the guild notes too often in short period would get you kicked from the server. Even if it would be possible to share the data you would still NOT be able to get the needed unit information and position of other players in realtime, which is good that it is not possible, to prevent botting, cheating and addons like Miat'S PVP alerts that alerted you before someone even short t you (from sneak)...
There were the same questions in the past several times and so far there was and is no solution I'd know.
But ask the other guild, I'm pretty sure they also don't own such an addon. Or if, it's made of whatever code and maybe external tools that might be forbidden by ZOs TOS.