There are some campaigns which accept Jr.Member in their campaigns like secondstrade but still i will say wait bit more and reach members rank as i also joined campaign when i became member because more options will be available then and a better payout.But if you still want then secondstrade campaign will be for you.
Well. if someone can join a signature from Jr Member. I don't think he should wait to become a member.
he can become a member or up while earning money. so waiting to become a member and not joining a campaign is not cool if you can follow the rules of a campaign and can maintain your post quality then you should Join signature for sure. and rank your account up while earning few satoshi. no matter it is low because you are anyway doing posts.