if I'm not mistaken, the minimum is members and for newbies you can't.
This really depends on the project you are joining. That is why it's good to read the instructions for each project before joining just as you have said earlier. Some projects do allow you to join their signature campaign as a junior member. I can see you got 2 merit already, work on your activity to become a junior member. With that you can now take part in some signature campaign. But always read the instructions as a guide for any campaign.
since have being on the forum here, i haven't seen any weekly bitcoin signature that accept junior member rank, the minimum rank i have seen is member rank which is also very scarce, just as you said the signature campaign which you can participate in as a junior member is bounty signature campaign which is as useless as bounty campaign, so if you really want to participate in bitcoin paying signature campaigns, you have to put down your head, read and learn from the forum and you will be able to rank up your account to at least full member rank so that you will have higher chances of being accepted in signature campaign, but if you are waiting for any manager to launch a campaign which will accept member rank, then you will have to stay long, and am not even expecting any manager to launch a weekly paying bitcoin signature campaign that will accept junior member.