When you make the group, you have to specify whether it is a Public Open, Public Closed, or Private group. Public groups are viewable on the the web by non-members. Here is what a public group looks like: https://www.zotero.org/groups/2205533/test_items_library/library
https://www.zotero.org/support/groups
(It's also possible to make your personal library public, but that's all or nothing.)