Everything about notebooks is covered in this link
From a sharing standpoint, the problem with X.nb.html is that it's static, rather than interactive in the same way as X.Rmd, where you can run chunks, edit them and run again, etc. And, of course, not everyone is interested in recoding a chunk, but would like to know what happens if you change an assumption or run the code on a different view of the data. That's what shiny is for.
Assuming your group is at all the same level, they can, of course, knit the Rmd file to create an HTML file, with, or without editing. Or, a knitted file could be posted to a website, using blogdown. For multiple authors, github is a great choice.
As far as learning markdown, there's always the option of coding in HTML, the awkwardness of which markdown was invented to relieve. If that's a dealbreaker, there's the Swiss Army Knife (complete edition) of document converters, pandoc, which interconverts among most formats which are known to UC Berkeley.