What is the lowest rank that can apply for a campaign?

[Gposas]

We see different campaigns allowing only from the rank of Sr. Members upward to apply, does it mean that lesser ranks are not allowed to apply for these campaigns?

[risatrakib]

Yes, it means they will allow only Sr. Member or above to join their campaign. But these rules are only for signature campaigns.
If you want to join any social bounty campaigns then you can do with any rank.

[Master107]

We see different campaigns allowing only from the rank of Sr. Members upward to apply, does it mean that lesser ranks are not allowed to apply for these campaigns?

Most of the time lower ranks cannot join signature campaign because the manager is just looking for the higher ranking. Number of posting is required as each participant will promote them through signature in each post.

Somehow there are signature that allowed Jr Member to participate. Meanwhile, do quality posting everyday to rank up soon.

You can join social media instead while waiting to rank up.

[Freemind]

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.