Zotero Groups allow you to share a library collection (folder) with fellow class members, your research group, or colleagues at another institution.
Once enabled, Group collections appear below your Zotero library in the left pane:
There is no limit to how many members may join your groups.
Be aware that, if you are the group owner, the data stored in the group library will be part of your overall storage quota. Other members of the group will not have their quotas affected.
Groups can be created within the Zotero pane or directly at Zotero.org.
First, make sure that all members of the group have Zotero accounts.