It is possible to share a mailbox folder, with another user, using the Zimbra Desktop client. The following article contains the steps necessary to share a folder.
1. Open your Zimbra Desktop Client by selecting you desktop shortcut.

2. From with the Zimbra Desktop, select the Mail.

3. All mailbox folders will be displayed in the left hand panel. Right-click the folder you wish to share and choose Share Folder.

4. The Share Properties window will appear. Adjust the settings appropriate to your needs. The below example shows the mailbox folder is being shared with an Internal User with a Security Role of Viewer. The standard acceptance message will be sent to the user. Click OK to share the Briefcase.