On Kudoboard for Business, you can upload a CSV with all your users. If you do this, you'll set a temporary password for your users, and you'd then send them a note letting them know they have access along with their login details (we don't send the email).
However, we typically recommend an alternative approach to onboarding.
Simply create a board for a special occasion (like a teammate's work anniversary) and invite your teammates to contribute. This will typically get users more excited about the platform and likely to onboard (vs. simply sending them access to a new platform without a clear call to action).
For larger organizations with multiple teams, you can upload a CSV with your "champions" who you believe are likely to create boards. Then, these champions can create boards and invite their teammates to contribute to a particular board (and those users can enroll as part of the contribution process).