Many forums will have different sections for different levels of members. For example you could have an introductory member that can access one set of boards, and then a paid subscription membership that can access premium content.
These various levels and Simple Machines Forums are called Membergroups.
To create a new Membergroup, go to Admin -> Members -> Membergroups -> Add Membergroup. Give the group a name, a Group Type and select permissions (which you’ll edit later as descibed below) and Visible Boards they are allowed to access.
Then edit the “Permissions” for the new Membergroup in Admin -> Members -> Permissions -> General Permissions -> and click “Modify” next to the new Membergroup to verify the permissions are what you want.
Then verify and edit which boards Membergroups can see in Admin -> Boards and over on the right click “modify” next to each Board and then click or unclick the appropriate groups under “Allowed Groups.”
You can change a member’s membergroup by going to the top Members area, and clicking on the member’s name which takes you to their profile, and under “Modify Profile” you click “Account Settings” and you can select their Primary Membergroup there.
When done be sure to test registering a test user and modify their membergroup to verify they are doing what you want (note you have to use an alternate email address for each test user).