Tim, I think that I may have sabotaged your thread. I am sorry for that. Let's see if I can get it started again.
My suggestions:
Get with the others and agree on a list of attachments. Have each person set their own prioties on what they must have.
For example:
If you live in snow country it is kinda hard to share a snow plow, unless you are near neighbors. If your are near neighbors this can work real well, as one person can plow both driveways if the other is out of town.
A mower may be used every week and it would be difficult to share back and forth unless you live close. Even then, since it is needed often, you might spend a lot of time moving it back and forth.
Other items are used only seasonally:
The aerator is not used very often and might share well.
Things that are used only for special projects and are very expensive could be shared well. Examples:
Trencher
Post hole digger
Different augers for the post hole digger
Stump grinder.
The trouble with the above is that they all have wear components. Here is where it would be good to have a maintenace fund. When a non-owner uses such a device, he would put some money in a kitty for repairs. Perhaps so much per hole, or so much per foot, or even hour of use. I have had the motors on both my stump grinder and trencher rebuilt because something got caught in the seal. Under $100 in both cases, but this could sour the co-op. Another option is "if it breaks when you have it, you pay for repairs". This is what Clint Blake and I use. This is simple, but if the attachment has been used and worn prior to your use, then it may seem unfair. Of course you can always work out a deal on any specific failure. Still if you know the rule, it makes things easier ato accept.
The grappling bucket and the 4 in 1 are often items of discussion. They are both excellent attachments, but each one is superior to the other at certain functions. They are also fairly expensive. I think these would be really good items for a co-op. They also don't have motors to fail and I think repairs would be fairly inexpensive.
These items might be owned in common, but if somebody moves what do you do.
Other suggestions:
Take a digital picture before and after a loan. This can save a lot of arguments.
In order to help balance things, Clint has purchased a 12 inch bucket for the back hoe. We both have backhoes, so we can move this back and forth pretty easily without stopping normal use.
Attitude: Some people get very concerned if their attachment is scratched, dented, worn, or even used. They should not be in a co-op. You have to accept damage and the fact that things wear out. I figure if my trencher is not worn out before I die, then I have not dug enough trenches for my neighbors.
I hope this helps, but it does seem to get a little complicated. I look forward to comments from others.
Bob Rip