To be in a Signature Bounty Campaign you will need to be able to use a Links in your signature and Jr. Member can't do that.
So you will need to be Member.
You need 60 posts and 10 merit.
Whose jr member can not follow signature campaign? Many projects allow members of the jr to follow signature campaigns. Only part of it that prohibits jr member follow signature campaign in its bounty. Because according to their jr members will only make spam.