What does it mean to be a co-author? What does acting as a "co-author" entail, and how common is it for first year undergraduate students to be added as co-authors to a paper being prepared by a professor?
Almost any published work has one or more people listed as authors: they are those people who made a significant contribution to the creation of the work.  If there is more than one author, they are called co-authors, and one might say they co-authored the work.  Usage like "become co-authored into a paper" would be non-standard and perhaps non-grammatical; one would instead say that a person became a co-author of the paper (typically for having contributed to it in a significant way).