I am PhD researcher; I interviewed my fellow PhD peers and one of those peers has experience with the type of research method that I hope to use. Can they be a co-author in my publication?
I have published a book, and one of my PhD D students shared authorship with me, the point is if he/ she is capable to write good scientific efforts for that book. Cheers.
Ideally not, to my knowledge. Participants cannot be authors due to various ethical considerations. It can/may go against research protocols. However please check your Institution's research policies for more insight.
One issue here might be co-authoring a portion of dissertation. Some institutions insist that dissertations be the sole work of the doctoral student who is authoring them.
My personal opinion is that it may increase the chance of bias, but if they are only used for the methodology section, and not writing up the actual results, then it may be alright, but this does depend on your university ethics policy as mentioned by Sachin Suknunan