As @risatrakib and @Master107 have said, typically bounty campaigns that require at least Sr. Member rank is to be able to participate in signatures, but that does not mean that any user cannot participate in other modalities such as Twitter, Reddit and others. If you look at the active campaigns in the forum, you will see that many users with lower ranks can also participate without any problem, but keep in mind that to participate, for example on Twitter, the bounty manager will ask you to have an adequate amount (always a minimum) of followers.