Currently, only users who are members of a board can be added to cards within that board.
This means that all board members automatically have access to all cards on that board, and can also work on them.

However, there have been repeated requests from users who would like more granular access control — for example, the ability to share or assign only specific cards with certain people, without giving them visibility of all other cards in the same board.

Proposed enhancement
Modify the permission structure so that it becomes possible to:

• Add users to individual cards even if they are not board members.
• These users would then be able to see and work only on that specific card, and not access any other cards in the board.

When such a user is added to a card:

• They would receive an email notification informing them that they have been added.
• They could access the card via a direct link, which they may bookmark for future use.

Possible implementation ideas

• These single-card memberships could appear in a “My Cards” or “Personal Cards” section for each user.
• To keep things organized, users could optionally categorize or group such standalone cards into folders or custom "mini boards" of their own.
• This would make it possible to work on shared tasks without exposing the entire board, keeping data access minimal and secure.

Why this is important
This feature would greatly improve privacy, data control, and collaboration flexibility — especially in projects where external contributors, clients, or temporary collaborators should only have access to specific tasks. It would enhance the feature "linking cards to other boards".
It would also enable more secure cooperation across teams or departments without needing to create separate boards for every limited-access case.