Collaborative paper-writing with non-LaTeX-using colleagues Background: I work in a field where the use of LaTeX is common but still far from universal. So far I have been lucky in that all my coauthors in the past have been fellow LaTeX users, so collaboration boiled down to creating a shared Dropbox folder with the .tex and .bib files we were working on. Now I'm starting a collaboration with two colleagues who use Word, so they have proposed to collaborate with Google Docs. There are various reasons why this is a bad idea, the main one being that writing this paper is going to require doing things that are easy in LaTeX but difficult and time-consuming in Google Docs (or in a standard word processor, for that matter) ---e.g., Greek letters for variables, assorted math/logic symbols, trees (in the graph-theoretic sense), or frequent crossreferencing in the running text of numbered examples. 

Conflict: One of my colleagues has already said he has no interest in learning LaTeX. On the other hand, I don't want to go hunting through the Google Docs character map every time I need to insert a non-Latin character. 

Question: Is there any collaborative writing software that allows including LaTeX tags and environments in a Word-like document? For example, when I'm writing semi-informal things like lecture notes, I can get by with markdown and then generate a pdf with pandoc. I don't know of any online services with similar functionality.
Excluding LaTeX-focused online collaboration services, such as ShareLaTeX, due to your new collaborators' preferences, I think that you have pretty much two major options, as follows:


Overleaf (formerly WriteLaTeX)
Authorea


Both online academic collaboration services support simultaneous use of LaTeX and WYSIWYG rich text mode, Authorea also supports some other formats, such as Markdown and HTML. Both services (to various extent) offer other nice collaborative features, such as data sharing, version control, revision notes and much more. Due to multi-format support, I would prefer Authorea to Overleaf, however, the final decision should be made upon a comprehensive comparison of both services across all available features and your detailed requirements as well as some testing.

P.S. Just for completeness, I will mention two other options. The first is to use blog engines that support both WYSIWYG and LaTeX (most of the major ones do: from WordPress to Jekyll). It's a decent option, but I would prefer one of the above-mentioned dedicated services, for multiple reasons. The second option is to self-host RStudio Server (or maybe a custom Shiny application), which would allow academic collaborative writing, using RMarkdown, but IMHO this is the worst solution possible, as trying to implement various needed features and solve issues, such as version control integration, would bring you and your collaborators a lot of headaches.