Brad, I have a joint-ownership post-hole auger and it has worked out very well so far. The neighbor who shares ownership with me has a very similar value system and economic level. I mention economic level because if you get into a situation where there are great differences, it seems there are different expectations about maintenance and repairs. Anyhow, here are my suggestions regarding joint-ownership.
1. I would not want more than one other partner. It makes sharing a lot simpler and prevents the "he said, she said" situations.
2. Agree up front that if you break it, you fix it, or to share all costs of repair. I like the first option. I broke a pto u-joint and bent the drive shaft, so I went to TSC and bought a replacement immediately. You can't put off repairs to shared implements.
3. If I want to loan the auger to a relative or friend, I ask my joint-owner if it's okay. If he says he doesn't want to do it, I honor his wish; case closed.
4. Cooperation is the key. If the other joint-owner has started a job with the attachment, you should wait until he is finished before asking to use it. Time of his job and breaks in continuous usage may apply, but unless he offers, don't expect him to interrupt his job.
5. Pick a place for the implement to stay when not in use. With my neighbor, we leave the auger with the owner who used it last. When the other owner needs it, he just comes and gets it. It helps that we are only 1/3 mile apart on a county road.
6. If there is a dispute about the implement, you need to have an up-front agreement how to handle this. For instance, each owner would agree to buy or to sell the implement to the other owner for 1/2 the current appraised used value. Sometimes discussing this issue up front will tell you whether the other owner will make a good partner.
I'm sure there are other issues, but these are just some that my neighbor and I have discussed informally. We don't have any written agreement, and truthfully, we've both far exceeded our ROI expectations, so the greatest potential loss in any dispute would be our friendship. No implement is worth that.