My family and I are enrolled in the Family Membership program and there are a few things to note:

If you ever want to convert bonus miles to status miles to upgrade a family member's card status, it is not possible. Since all of the miles earned by family members (except the master account, of course) automatically go into the master account, these family members always have a "0" balance in their account total.

Also, my wife and daughter were recently traveling and wanted to upgrade their seats at the check in counter. However, they were not able to because neither had any miles in their accounts, and since I wasn't traveling with them, they wouldn't allow them to use our Family account.

Seems that there are some tweaks that need to be done with this program. In general it's nice, but there are some issues that actually penalize you by joining.