Trello – Cannot Leave Trello Board

If the user is in the same organization as you and the board is part of that organization, you can automatically add them to the board:

1. Click the Add Members button and they will be listed under the current members section.
2. Click a user and select Add to board.
3. They will get added immediately, without going through the email / accept process.

There's more information about it on our blog: Improving Organizations: Adding Members and Self-Join

If they are not in the same organization, you will have to go through the normal invite process.

The way I found to do this, is by changing the Board permissions so that only members can make changes to the Board. People in the Organization can view the board, but they can't become members unless invited to the board.

This way I can invite people to the Organization, let them view the board, and still, they will not be authorized to change anything on the board, and can't join the board without my specific action.