Hello Guys,
I finally figured out a solution. Please left click on the Ribbon and click on customize ribbon bar. Then click on "Review" and then click on New Group button.
Once done, please choose "All Commands" from the list "Choose Commands from" and search for
"Share Workbook" from the list. Once you have identify it, click on "Add" (This will add the command in the new custom group". At last, click OK.
P.S - Sorry for my bad English, the option was grayed out because the workbook was never "Shared" to be "Unshared". Hope the above will solve your problem.