It totally depends on what some key factors like how your bounty campaign should look like rsp. what you are giving bounty for, how large your community already is and how well you're connected with it etc.

You don't necessarily need an (external) bounty manager, you certainly need someone who is responsible for organizing and tracking it but if you're doing something on your website, a bounty manager or team isn't needed. you might want to consider advertising the bounty program in such groups. Some of them also promote projects they don't manage (for a fee of course).