A grantee is the person on the retention document with "title" to the number plate - only the grantee can add/change or remove nominees and complete the cherished transfer.
The grantee is the person with legal entitlement to display a number plate, although you can never "own" a number plate, being the grantee is the equivalent.
The grantee can assign the number plate, add or change a nominee and extend the entitlement period.
It's not possible to add or change a grantee. The grantee is the only person "entitled" to the number plate. They would have been the registered keeper from the donor vehicle where the number plate originated.