How do I add a slash command for certain users to use?

You can add permissions to a certain slash command by using the command.permissions.add() function. Your code will look like this:

const command = client.application?.commands.fetch('YOUR_COMMAND_ID');

const permissions = [{
    id: 'ROLE_ID',
    type: 'ROLE',
    permission: true,
}];

await command.permissions.add({permissions});

Source: https://discordjs.guide/interactions/slash-command-permissions.html#role-permissions