I'm an undergraduate at an American university. I have been working with a collaborator (at a different university) on a paper for the last few months. I recently set up a meeting to discuss the work with a professor at my institution who is an expert in one of the topics related to this work. In the meeting he clarified a lot of my ideas.
The discussion was only an hour or so, but as of now I do think at least one of his suggestions will make its way into the final paper. Thus, it seems clear that he should be an author. I'm wondering, what is the typical way that this subject is brought up? Should I just ask him at the next meeting, "Would you be interested in being a coauthor?" Is that the typical way this conversation goes? This is the first paper I've worked on where the authorship hasn't been determined at the very beginning, so I really have no idea how this works.