I'm an undergraduate submitting my Resume for a variety of internships and need advice about how (if?) I should include my contributions to a manuscript that my group intends to publish. This would be my first publication so I leave it off I feel that I would really be cutting myself short. I mention my position and the work I did, but I feel it won't carry nearly as much weight if I don't mention that it was in support of a paper.
As a student, it is entirely appropriate for you to list manuscripts "in progress." However, I would make sure that the manuscript is actually progressing before listing it on your CV.
If it's in preparation, you then list it as you would any other publication, except where it's published (journal, volume, page, year), you instead list where you plan to submit it (if you know that):
Author List (including you). Title of manuscript. To be submitted to Journal X.
or, if you don't know where it's going to be submitted:
Author List. Title of Manuscript. In preparation.