In order to assign apps to an enrolled device running iOS 8 or OS X 10.10 Yosemite, you need to invite the user to receive the VPP Apps that your organization buys. When you click ‘Apps’ on the sidebar, you will see a tab for VPP. There is a VPP Users tab that will allow you to manually send a VPP invitation to any device.
Users will receive a popup on their device to accept VPP apps. The user will accept the invitation on his or her enrolled device and all assigned Apps will be linked to the Apple ID that’s currently signed in.
Volume Purchase Program apps assigned to devices running iOS 9 or OS X 10.11 El Capitan do not require an invitation or Apple ID for app deployment.