I could have swore you guys always called them "aggs" like "aggro", can't say I understand the "add" term, but I get what you're talking about in chat. All news to me.
It's an old MMO-RPG/MOBA term, it means a bunch of AI controlled minions or baddies, fodder.
Aggro is as well. Tank, support, mobs, AoE, DPS, HPS, etc... most of the terms people are used to are from MMOs and, by proxy, table top RPGs, namely Dungeons and Dragons. Some of these terms are 30 years old. As a D&D player for 25 years and an MMO player for 15 it's funny to see how in meshed they've become in gaming culture now.