Is it possible to invite external people to particular boards, without inviting them to SP Site Collection?
Is it possible to invite external people to particular boards, i.e. not inviting them to the SharePoint site but to a board within Kanbo?
In the current version the Security of KanBo is based on the security of SharePoint, so when you add a user to a board KanBo adds this user automatically to the corresponding User Group in SharePoint. The workaround is used to remove the user manually from SharePoint after he has been added to the board. The user will lose access to all documents, because they reside on SharePoint - except you are using a different library for this board (Document Sources management).
Learn how to invite external users by following instructions here: http://community.kanboapp.com/topics/55-invite-external-users-to-kanbo-office-365/