I'm new to this but what are signature campaigns and how do you join them?
It's more of a get-paid-to job where you will earn some few cents for every post you make. Basically you are advertizing a service in your signature space thus the name signature campaign.
Different campaigns has different set of rules and different ways of enrollmen procedure. You can join as a newbie but that will not really be worth the time and most campaigns don't prefer to hire a newbie or a lower rank member to advertize them.
Best explanation so far, in addition it can also be abuse like people spamming every thread without even the need to post what they say(more like nonsensical post) but the reason they do it is just to meet their signature campaigns' quota.