I'dd add some brown in the form of leaves, hay or lots of shredded paper/cardboard. If you can keep the pile moist and add small amounts of worms they'll take to it and start breeding once it's cool. If it's still hot you'll want to leave it till it cools to around 80F.

It's a good material. If the coffee stays moist and on it's own it tends to mold or produce some sort of fungus. If you can mix it with carbon and some water it'll compost well and makes a good bed for worms. If you want it to sit for some time you could add veggie scraps or other food and leave it for 6 months or a year while the worms eat through it.

Good luck, you've found a good source of biomass.