How to fend off someone putting their Name on my work? I'm a postdoc and a guy who used to be a part of our group dropped by today and said repeatedly that he wanted to collaborate with me and that he's too busy with other commitments to do any work. I suggested a project where he could write up some of his recent experiences in the area (I know of a publication that did something similar to this that we could use as a framework and am working with another man in our group on this), and he had no interest in that saying instead he was more interested in the data analysis I'm doing.  

The message I got was "I don't have time to do anything, but I want to be a co-author".
He was "generous" enough to say he "doesn't need to be the first or second author".

How would you handle that?

During my PhD I had one of my supervisor's former students do this to me. He got 3 publications out of me, which he didn't understand well enough to present at a conference (he insisted that I create slides for him and write a word-for-word script for him to deliver - the conference was in his area of the world).  I eventually put my foot down and refused to have him as a co-author again (my supervisor was pushing to put his name on a 4th publication, I think she wanted to help him get tenure and / or was repaying some debt to him with my work).

I don't want to spend time trying to defend my work from people who didn't contribute putting their names on it, but I'm not sure the best way to get rid of him. My PI thinks quite highly of him.

I'm going out with my PI casually for dinner tomorrow and am planning to bring this up and ask him to be explicit about his authorship policy (my PI reserves the right to determine authorship of work from our lab).  I don't want to seem whiny, but also don't want to spend time / energy worrying about this.

Am I overly anxious about this?  Maybe it doesn't really matter as long as I'm the first author?  If that's the case, I'm tempted to just put everyone I know on it (I don't want to reward this guy for being obnoxious).
Your situation sounds messy. I think most will agree that the basic prerequisite for co-authorship is intellectual contribution to the manuscript . To add authors for thanks is common but really not a practice to endorse. If someone comes to me and asks to be a co-author, the obvious response would be: "What will be your contribution?" If there is no clear contribution other than, say, "Read the manuscript and provide input" then the answer should be "No, thanks". In your case, it sounds as if there is no major point in even having a discussion.

In fact, your case reminds me of a colleague of mine who ends up in all sorts of strange situations. His problem is that he is too nice and does not know the word "no". I realize it may be hard to set up strict rules for these things if your immediate surroundings do not subscribe to them, but I think you should know that you have solid grounds for being more strict about adding unnecessary co-authors or letting people into a project without a clear picture of why.

If you are first author on a paper, it seems to me you will also be the person deciding how the work should materialize. I think you need to make your own mind up where you want to draw the line. you will benefit of making a decision on how you want things run. If you want to deviate from this later on, then it is still your decision and no-one else's. I think this is what you (and most of us) really want - to be in the driver's seat. So follow your instincts, they seem sound to me.