Hi Jaime,
each of them should have access just to their own lists of suscribers and they own bulletins. Maybe an Administrator profile could access to all lists.
Unfortunately, that’s not possible. With Members, you can set the user’s permission to access specific MailPoet admin pages, but not specific lists only from the Lists page.
The best option, in this case, is to have different websites with their own MailPoet install.