to sign a form at a government office to say that you do not have a job and that you want to receive benefit (= money paid by the government)

to agree to become involved in an organized activity

[ + to infinitive ] mainly US I've signed on to help at the school fair. 我已报名在学校交流会上帮忙。

to sign a document saying that you will work for someone or do a particular job or activity

She's signed on with a temp agency. 她和一家临时机构签约了。