Your account administrators can edit users' access rights. Admins can edit their own access rights by clicking their profile picture in the upper right corner, then User, and then Edit under the title Your Access Rights at the bottom of the page.
Other users' access rights can be edited by going to Account page (click profile picture in the upper right corner and Account) and Edit Users and Access Rights. There you will see a list of all your account's users and their access rights. Click Edit next to the user name, and you will see the below screen.
Note: Users cannot see the categories they don't have access rights to. For example, if a board member has access rights only to Board Meetings, that user will go straight to board meetings page when signing in, and the user will not see any other categories in their account.