Young people, in tenure track positions, feel they to have to collect as many authorships as possible. Questions like “Will I be a coauthor?” and demands as “I have to be a coauthor” are part of daily conversations in science institutes.
But not only junior scientists are eager to boost their cv’s with authored papers. Seniors use their social status to fight themselves into the list of authors.
In a number of groups the list of authors is not discussed but imposed upon. The boss decides. Him being the last author is an axiom. The issue of free riders imposed by the boss, gives rise to a lot of frustration on the side of the first author.
The question of coauthors only arises when the text is mature enough to be a concept for a paper. Or when there is agreement on what material should be in the paper. Discussing authorship earlier is a guarantee for friction. A common mistake made by Ph.D’s and postdocs is to circulate concepts of papers with an incomplete list of authors. If you are the first author and not yet sure about the complete list of coauthors put a placeholder in the list of authors: “Your Name and Rest of Authors”. Somebody that spots a misled copy cannot (yet) complain as he might think he will be part of “Rest of Authors”.