What is a social media campaign? It's a campaign for social media for example is twitter and facebook. If there is a new project that's going to open and ofcourse all they gonna do is to promote their project thru facebook and twitter to increase their exposure. Uhm it's like you're going to adverise their project to your facebook or twitter account.

Can newbie join? It depends on the rules of the project some project accepts only jr. Member and above but there are some that they just accepting people with 500followers and above regardless the rank in bitointalk forum. You can read all the rules and regulations including how you will apply and the payments at the first page of the projects thread. I hope this will going to help you and also welcome to the forum

The requirements vary based on the campaign but if you have a  strong social presence you should be able to leverage it for some money. I want ot say 5,000+ twitter followers or 500+ facebook friends are good starts for ya hope this helps!