I would tell them how their information would help enrich the research paper by providing detailed first-hand knowledge of a certain aspect of the topic and that if they can be quoted and use their name in the text of the paper it would lend a note of authenticity to the points being discussed.
MLA in-text citation style uses the author's last name and the page number. If the source doesn't include a page number then only use the last name of the author.
