Welcome to the forums,
-How about sending the standard email? When they accept via the buttons given at the top of the email it'll auto add the share to their list. (see attached pics of the process)
-If you have terminal access (I kinda doubt you have this) there's
Zmmailbox - Zimbra :: Wiki which can be scripted to do that work. Perhaps 01 could setup a portal for you to do this or something.
-You can also vote for
Bug 7473 - Share management and discovery (have a page in preferences that show all the shared items available to a user)