You may want to have a quick chat with an employment lawyer in your country before you make your first hire. Employees in the US are owed minimum wage by law, along with FICA taxes, unemployment insurance, etc. Interns are not a special case. There is not a legal way to have an unpaid intern who does marketing work for your business, nor to pay them less than minimum wage if their "profit sharing" happens to add up to less than that.

Have you considered running an affiliate program?