When you have the boardroom activated, you will need to have board members in it. The only ones with access to the boardroom are the ones who have the explicit "board member" role.
Note: Company admins don't have access to the boardroom automatically. You need to assign the board member role to them for you to have access.
There are 2 ways of adding board members:
How to add a board member role:
Navigate to connection management
Click the three dots on the user you want to give boardroom access to or select multiple persons by checking the check box
Click Assign Roles or Manage roles
Select the role "board member" and click Assign roles
The persons have now a board member role and can see the boardroom in the left-hand side navigation.
How to invite board members
Navigate to settings > connections
Click the "Invite People" button
Choose the role of "Board member"
Add the emails of your board members
Click invite