Hi! You need to have Microsoft 365 Account to use Viva Engage, as the user needs an account as it's mapped against AzureAD/M365. This is due to Yammer/Viva Engage being in Native mode.
Viva Engage is designed to be a work tool. There is a way to have guests from outside your company but those users need to be M365 work accounts, so this does not accommodate a family member use case. There's no "Viva Engage personal" like there is for Teams.