We use Trello in our office for keeping track of projects etc and it proves very useful.
However, we only use it internally. After engaging in about 100 emails with a client over a week it has prompted me to possibly setup a 2nd client external board. Where both myself and clients can add/edit lists and cards.
Is there a way you can do this, but only restrict access for the client to a specific list including its cards. But obviously we would want clients to access or use other lists.