You cannot as you got no access to the other UNGROUPED players position, nor their nameplates, nor their model in the 3d world.
You can only change your own reticle as you look at them (have them below the reticle), or the UI frames showing the currently selected target.
Search in the API txt documentation files (
https://wiki.esoui.com/APIVersion#100033 -> "API TXT Documentation") for GetUnit* functions, and use "reticleover" as unitTag. unitTag "player" is your own character.
https://wiki.esoui.com/UnitTag