Sorry, I had edited my post above and forgot to change the upper text about "not possible"! My fault.
It is possible as I found out later by Votans addons. What I meant was that it is not possible to show floating marker icons above heads of enemies or NPCs or group members (group members can only get the shown arrow replaced by another texture, that's all).
Oh yeah I missunderstood your idea then. You mean to add the textures to the group UI "frame" where you see the whole groups health etc.
Check the code of addons related to group frames then to learn it at best I'd say:
e.g.
https://www.esoui.com/downloads/info...oupFrames.html
https://www.esoui.com/downloads/info...nitFrames.html
ZOs named them unit frames, so the source can be found here:
https://github.com/esoui/esoui/blob/...me/unitframes/
https://github.com/esoui/esoui/blob/...frames.lua#L10 (ZO_GroupUnitFrame)