User and Share (folders) and their permissions can be created, managed, and deleted via the My Cloud Dashboard. If you have not done so already you should read the My Cloud User Manual (http://support.wdc.com/product.aspx?ID=904) or the My Cloud Dashboard Help. Both have sections that detail how to create Users and Shares and how to set access permission (Full Access, Read Only Access, No Access) to those Shares (and their contents).
Officially the single bay My Cloud units do not support third party apps/modules. The My Cloud Mirror and the other more expensive multi bay My Cloud units do support additional third party apps/modules.
WD has a My Cloud Developer section that you may want to visit and review for creating “apps” for the multi bay My Cloud units:
http://developer.mycloud.com/MCDSDKindex.html
Unofficially, and absolutely not supported by WD, it may be possible on the second generation v2.x firmware single bay My Cloud units to modify the firmware to allow for third party apps/modules. See the following thread for more information:
https://community.wd.com/t/wd-mycloud-gen2-enable-apps-install-tab-apps/177885
