I love apples and have some young apple tree guilds developing.
Key guild components:
Comfrey - aim for one of the sterile bocking varieties and propagate by
root cuttings. Comfrey deep mines minerals from the soil and returns them to the root zone of the apple tree. Regularly chopping leafy material mulches the surface of the soil, and the flowers attract pollinating insects. A well established patch of comfrey will also do an excellent job of shading out and suppressing grass, which competes substantially with apple trees.
Deep taproots - I like thistle family plants here - globe artichokes and cardoons for example. Their root zone is much deeper than that of the fruit trees, they have an edible yield and their leaves again can suppress grasses once established. Another pretty good mulch material if you let the old leaves fall.
Creeping ground cover - again, this is about limiting opportunities for grasses to establish. I have strawberries running under my trees and it looks like I'm due for a bumper crop this year.
Berry bushes - I've got some redcurrant under/around my apples. They are a new experiment but I'm hopeful they will work well. Again, I'm looking for a dense leafy shrub that will shade the soil and keep the grass at bay.
Various herbs/alliums etc - I've got a few odd alliums in as well. Some walking onions, a few chives that I let self seed, some creeping ground covers.
N-fixers - I don't have any at present, but would be looking for some perennials. I've heard good things about alder, coppiced regularly to ground level. Lupins can work if you can get the
roots innoculated well.
Honey locust would be good to mix in - looks for a thornless variety, ideally one also bred for high sugar content in the pods.
Getting established
Wood chip mulch is amazing for this. It tipped my from dense grass sod to rich, dark, fertilise soil full of moisture and earth worms in under 6 months. We get loads dropped off occasionally by tree surgeons. Lay it down thick (like 4 to 6 inches thick) and when you need to plant just scrape it aside to get to the soil. I'm hopeful that once everything is established and holding the grass at bay then I will only need to very irregularly top this up. I've noticed that the strawberries really love growing on the wood chip. They like being elevated from the mud a bit I think so the fruit and leaves stay dry and clean.