From MobileMe Help:
Converting someones account to a Family Member account
If you have a Family Pack account (or plan to purchase one), you can convert someones individual, email-only, or even an expired account to be one of your Family Member accounts. After the account is converted, all files (including email) associated with the original account are accessible from the new sub-account.
To convert an account, you need the MobileMe member name and password for the account.
NOTICE: MobileMe no longer offers email-only accounts. However, account holders who have email-only accounts can continue to use, renew, or convert them to Family Member accounts using the instructions below.
To convert someones account to a Family Member account:
If youre not already logged in to MobileMe Account, go to
www.me.com, log in, and click the Account button in the toolbar at the top of the window.
Click Account Options in the sidebar (on the left).
Do one of the following:
If you already have a Family Pack account and have at least one Family Member account available (that is, not already assigned to someone else), go to step 4.
If you already have a Family Pack account but all four of your Family Member accounts are assigned, you need to remove an existing account before you can convert someones account to one of your Family Member accounts. Click Remove next to the account you want to replace.
If you dont yet have a Family Pack account, click the Upgrade button (if you have no upgrades yet) or the Add More Upgrades (if you have other upgrades). Then follow the onscreen instructions to upgrade your account to a Family Pack account. For more help, click the link below.
Upgrading to a Family Pack account
Click the Set Up Account link to the right of an unassigned Family Member account.
On the Set Up Subaccount page, click the Convert an existing account button.
Enter the member name and password for the MobileMe account that you want to convert, and then click Continue.
If the account information you enter is for a subscription that is still within the free 60-day trial period, the trial period ends as soon as the account is converted to a Family Member account.
On the Account Converted page, click the Print button to print the account information for your records. When finished, click Done.