Adding users to a Managed account works exactly the same as adding users to your own Aero account, except you will do it from the Managed account.
Switch accounts so that you are logged into your Managed account.
Create a new team member just like you would normally. If you are adding someone who is already a team member in your main Aero account, make sure to use the same email address for them so that they will be able to switch accounts without logging out and back in. Users from the main Advisor account, after the first advisor user, will count against the total users for the Managed Account.