...
Once inside the board, click on and on “Share” right after.
...
On the modal below, you will be able to share your board with:
My organization: Any logged user of your site.
Group: You can choose one or more groups that you have access.
User: You can choose single users of your site.
Private: Only you will have access to the board.
...
After each selection, click on “Add” to include it to the shares.
...