@Matthieu Bonnici when a user is added to a card as a member, they automatically "Watch" the card. There is also an option when you add a comment to watch the card, which is ticked by default but you only have to untick it once.
If you're watching a card, you receive notifications about everything that happens, which makes Trello's notifications completely unusable in my experience (and from what I have seen most people just turn off Trello notifications and use another tool like Slack for internal communication, leaving Trello to be more like a whiteboard/planning tool than a communication tool).
You can use Butler to unwatch cards you're added to, but I'm not sure if this will unsubscribe the added member or if it can only be used for the person who enabled Butler, the rule is something like:
When anyone is added to a card unsubscribe from the card
That's the only rule you can create with the power up -- it used to be possible with the Bot to say:
When anyone is added to a card unsubscribe the member from the card
and so long as the member had authenticated with Butler, it would unsubscribe them.
I created an integration called Benko Board which allows you to use Trello as a Gmail client, but it also unwatches any card you're added to anywhere in Trello:
https://trello.com/integrations/#benkoboard
It's installed for a user, not a board, because it's not a power up, so once each person installs it, it will work in any board they are added to.
You can disable the email stuff and use it only for the purposes of unwatching cards you're added to which, imho, makes Trello's notifications actually usable because you'll only get notified about being added to a card, mentioned in a comment, and removed from a card (and of course if you go and watch a list, board or card yourself, you will still get notified, but the default is to have far fewer notifications).
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.