Should I add a co-author? I am a Phd candidate. I'm writing a journal paper that is, in part, a review of the current regulatory landscape use of a particular technology.

Sarah is involved with an unconnected voluntary project which has been funded by a charity to use the technology, with a condition that the learning from the project is shared publicly. Sarah's correspondence with the regulator is very relevant to the paper I am writing, as they are charting new territory, but she is reluctant to share information as she is starting a consultancy business on the back of her experience with the project.

I think this information should be in the public realm (as did the charity who funded the project), so I want to find a pragmatic approach to getting it out there. Also, although the information would only make a small part of the content, my paper would be incomplete without it. 

In other circumstances I would interview her for the information and acknowledge her contribution to the paper; there are others whose experiences I have written about in a simialar way and acknowledged in this way.

But in this case, I am considering offering Sarah a co-authorship on the paper in exchange for the information. I would be the lead author; Sarah would be added after my supervisors.

Is this an ethical / appropriate use of co-authorship?   Sarah's contribution to the paper would be small, and perhaps not normally worthy of a co-authors credit, but without it the paper would be incomplete.
As you wrote,


  Sarah's contribution to the paper would be small, and perhaps not normally worthy of a co-authors credit, but without it the paper would be incomplete.


The recommendation is to add her to the authors of the paper. I think that little things would be the gem of your work versus the similar works and you don't neglect the small valuable information with her experiences through working with the technology. 

Moreover, adding Sarah as a co-author doesn't reduce the impact of the paper, but it also promotes the validity of your work.