Miles are pooled for useage within (and only within) the group, everything else remains seperate.

Depending upon sales, it can be worth buying miles and using them to upgrade a WT+ fare, rather than buying a CW fare outright. Possibly even buying enough miles for a CW redemption (remember that miles tickets are flexible). Each person in the household can buy 19k miles/year (there was recently a bonus too). You can have up to 7 people, including yourself, in the account.