How to effectively write a paper of N>2 co-authors? We are writing a manuscript with 7 people. I am not sure what is a good tool to write a paper collaboratively. Other than overleaf or sharelatex which cost money. 

What do you do if you have technology inclined collaborators?
Git! Or more generally, version control systems, if your co-authors are technically inclined enough to know/learn how to use one.

If not, probably... write the text in google docs first and designate someone who will do the formatting in the latter stages, and decide what to cut to make the page limit.