Updated for 7.0
Go to User Details of selected member (see: Search Members).
Select View Details on the top left of the pop-up box. This will take you here:
In the horizontal bar, select Management. This will take you here:
Type in the name of the member that you would like to add to the account.
Then, select the green button just to the right of the name.
Your page will reload and the member will appear within the Managed Accounts box.
These are samples.