Both Managers and Accountants can be hired within the "Managers" section of the "Menu".
What's the difference you ask?
Managers run businesses for you. They are accessible early in the game, and allow your profits to accumulate without tapping them, and continue to earn your Cash while you're offline. However, you always need to start your initial investment before they will keep it running for you!
Accountants provide discounts for your Investment prices, and help you buy larger number of the specific investment they are managing. Accountants are introduced later in the game, and are necessary to reach the higher levels of AdVenture Capitalist success.