You can invite users who have not accepted the initial invitation to join Apple's VPP.
- Navigate to Apps & Books > Applications > List View > Purchased for applications or Apps & Books > Books > List View > Purchased for books.
- Select Manage Devices ( ) from the actions menu.
- Select separate devices or choose to act on all listed devices and select Reinvite from the top menu.
- Confirm your choices.
This process is not necessary with the device-based VPP system because they system does not use invitations.
For an outline of the support for the managed distribution method by Apple IDs, see Managed Distribution by Apple IDs Overview.