There is no rank requirement for bounty campaign but once you try to apply for signature campaign then there has rank requirement. And it is totally depend on a campaign manager and project team agreement.

If you want to work on a signature campaign, then always keep an eye on the services board. There are sometimes full member slots available. If your account reputation and posts and merit points are at a standard level for 120 days, then the manager can accept you. but if you want to work in Bounty Campaign then you can join easily if you have the requirement social media accounts. Bounty is all about social media marketing so there is no forum rank requirement.