Zotero's Groups feature allows you to share references with other Zotero users online. It's a great way to work on collaborative research projects.
You now have two sections in your Zotero collections pane: My Library and Group Libraries.
Personal and group libraries are entirely separate, and changes made to items in one library do not affect the other. You can drag items back and forth libraries to copy items.
If you need to have a group folder with shared highlighting and annotations there are a couple of ways this can be accomplished. Basically, any markup on a PDF that is shared within Zotero should show that markup to other group members. One free tool for doing this on your desktop is Foxit. The iPad and Mac app Papership has a pro version that allows for markup as well.