1. To share Zotero libraries, you will first need to register for an account on the Zotero website. You will then need to sync the references in your library with your online account. Enter your credentials on the Sync tab of the Preferences menu, then click on the green arrow to sync to your online library. See the Syncing Zotero tab on this guide for more information.
2. Log into the account you just made. Click on Groups, then Create a New Group. You can then name your group, specify its permissions, and add members. Click on the green arrow again to add the group to your library.
3. You and your group members can then add references to your own standalone libraries and sync them to the online library. Feel free to add collections as needed. You can also interact with the group via the Zotero website.
Here is my group library in the standalone program:
And here is the same group library in the online library: