Addons cannot change the 3d world. They can add some markers like textures (HarvestMap) or rays from the top to show the group members (Lights of Meridia).
But they are not allowed to change character appearancve or visiblity or whatever.
It's either an ingame setting in the video settings which does this somehow or I'd be interested which "acknowledged" addon would be doing this.
There is an memento which can make players look a bit invisible , that's all.