If you're on the basic plan, the private and shareable boards are not part of your plan, unfortunately. However, there are two options:
- Drag and drop your private and/or shareable boards into your main boards section. Here you can use your board although the board will be visible to all team members.
- You can always upgrade to the Standard or Pro plan.
By the way... If you upgrade to the PRO plan, you'll receive UNLIMITED guest users :) Take a look at the pricing differences.