I started a few group libraries lately and I want to be able to know who added what new item to keep track of what everybody has read, who is working with what literature. Right now the only way to see who added what is to click on every item on the website, which is a big loss of time. It appears when you click on the home page of the group, but only for the last 10 items, then it doesn't show anymore. It would make it more collaborative to be able to see that from the zotero window, by having the possibility to add the column "added by".
Also different people are responsible for different parts of the building of the library, so it helps with knowing how far everybody is. It also helps further discussion. The information is there, it would just make it more practical to have it more easily available!