Ask yourself this - would they be interested in taking control of this account at any point of time in near future?
If the answer is No, then they are fine with you managing their investments on their behalf, where you have full control. In this case, create a Family Account for them, under your Kuvera login.
If you're not sure, then it's best to create a separate Kuvera account with their email ID, complete registration, and send a Manage Account request to your Kuvera account. This would make their Kuvera account a Managed Account.
