1/ You need to be on the quest "Equine Retrieval" (Part of The Norwington Estate questline, start with "The Norwington Festival", Alliance only)
2/ Summon a 2 places mount
3/ Click on Rose
Now you have Rose following you but unlike Cooper, Rose npc will not be here anymore.
4/Leave the Norwington Estate
5/Come back to Rose initial position.
Rose npc will have reappeared, repeat 2-5 to have an army.
Unlike Cooper : Rose WILL ATTACK her ennemies if she is hit first, so can you make her body pull the mobs, or if the mob is doing an aoe all your army will help you.
Also they are not walking in formation.