Assign Products to Client Contacts
In WHMCS there is a provision of adding Client Contact / Sub-client, when a contact is created by a client, he should also be allowed to assign domain / produces / services to a individual sub-client and choose the email which need to be sent.
This option could be seen as multi - reselling, where a reseller can purchase domains and services for a particular client and assign a individual domain / product to the client.
Sub-client could be able to manage the products / services assigned to him as individual.
All billing will be done to the main client only.