Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Which type of subscription is it that you have been invited to join? You mention 365 Family but that one would normally be for individual (home use) products. Business products are set up with business e-mails and require other licenses.
Note that if this is really a 365 Family invite, you can't use an account which you are already using for other purposes, for example, if the account is part of a family group. You'd have to make use of some other personal e-mail account. You probably have to ask for a new invite.