So, the five user limit works just fine for the nuclear family. Currently, each member has a live.com address. I can find no information on how one adds the additional accounts, and if one can, if one does not have a custom domain.
If you have a custom domain, do you need to specify the new domain email as well as the existing account, in order to add it as an alias on that account.
If you do not have a custom domain, can you link the accounts at all?