When you invite someone to your brand you can choose their role, and each role sees a different part of the platform.
Media buyer
Doesn't see cost, revenue or profit information. It's the right role for an agency or for someone who only manages your ads: they can work with ad metrics without accessing the business's sensitive numbers.
Inventory manager
Only has access to the Inventory module.
Admin
Full access to the organization.
To add someone, go into the relevant organization and add the user, choosing their role in that same step.