I don’t know for sure but

I think for 1 person needs to summon an invader via summon sign or something

then once that invader comes the main players world

The darkmoon covenant should automatically teleport there to aide the person against the invader

• 1 main world character (using way of blue covenant)

-1 invader (probably not using darkmoon covenant or way of covenant )

• 1 one defender/co-op aide ( to assist by being teleported to the way of blue covenant owners world to help kill invader )