Zotero Groups provides a method to collaborate with other Zotero users. There is no limit on how many members may join your group, and your full storage subscription is available to both your personal and group libraries.
To create a new group, click on the "New Library..." icon located next to the New Collection icon in the top left corner of Zotero. When you click on the New Library icon, select "New Group" from the drop-down menu (demonstrated below).
When you select "New Group," you will be brought to this page on Zotero's website where you can create a new group.
There are three different types of groups for Zotero users: Public, Open Membership; Public, Closed Membership; and Private.
Once you have created a group for your project, you need to invite members. To do so, go to the Zotero website, click on the Groups tab, and then on the "Manage Members" link. On this page, there is a link that will allow you to invite users, so be sure to have the emails they used to create their Zotero accounts handy!
To add items to your Zotero Groups, simply drag and drop the items from your Zotero Library into the desired Group Library folder. If you would like for group members to see changes to the Group Library, make sure that they have enabled data syncing.
The settings to manage who can edit the group's library and edit file attachments are located on Zotero's website under Groups, then the group you're using, then Library Settings, then Library Editing and File Editing.
There are two main options available here:
The most likely reason for this is that the owner of the group has filled up their quota of free space. The owner of the group can check on this by going to Zotero's website, logging in, clicking Settings, then selecting Storage.
If the owner has used up all of their storage, then they have two options: