To share references you and your group members will need to create an account at Zotero.org. Put your Zotero account's Username and Password into your Zotero desktop Preferences under the Sync tab.
Back at Zotero.org create the group library and INVITE others to join the library using the email address they used to register with Zotero.
Save your references into the group library. When you sync to the Zotero server, the reference you added will be uploaded to the Zotero server. Then when the other group members sync, that reference will show up in the group library folder in their Zotero library.
Your group libraries will appear in your Zotero desktop application underneath your personal collections.
You can share Zotero references with colleagues using EndNote or vice versa. Both programs can read and write a standard citation file format called RIS, which means references can be converted back and forth with a few clicks.
Zotero to EndNote
EndNote to Zotero