Q:

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?

A:

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.

A:

Short answer: it is a pain, unless you can all agree to use a single way of working, which is unlikely. I will describe my own workflow below in case it gives you ideas.
In my experience, each author wants to use their own tools. Depending on the field this is usually Word or LaTeX. In biophysics, almost nobody uses version control and most of the people I worked with are not willing to learn it.
I am not saying you should do what I do, or claiming that this is the single best way, but my workflow is as follows:

I write the paper as very basic LaTeX (almost plain text). I use version control.
I paste it into libreoffice and do some basic formatting (otherwise some co-authors will start "correcting" the formatting). I email everybody the draft of the paper.
Co-authors suggest changes, all at the same time. I ask them to highlight or track changes and, importantly, leave comments and tell me WHY they propose a change. You'd be surprised how many people make changes without letting you know why.
I manually merge the changes of all authors into my version of the paper, and deal with conflicts (if necessary by discussing via email with the co-authors in question).
Repeat from point 2 until done. For me, typically 2 or 3 iterations are enough but with difficult people this number can go up dramatically.
Convert paper to final format (decided by journal requirements). Make sure you do this only once, at the very end.

If this sounds complicated you are right. But to me this is the least complicated way unless everybody agrees to use the same software and version control (which is rare). Especially if reference management software is involved it can become a terrible mess when co-authors start to add/mess with references.
The main advantages of this method:

There is a single current version of the paper and I (as first author) always have it.
I can use version control, even if nobody else does.
Everybody can use whatever tools they want.

I found that one person being in control of the writing and merging avoids endless discussions and ending up with multiple versions of the paper.
If people take too long to reply I send them an updated version with the others' corrections: this will show them that others have done their job and hopefully incite them to reply as well. Also, some of their potential comments may already have been addressed in the new version.

A:

Use Google Docs or a Wiki (see http://www.wikimatrix.org/ )
Agree on a detailed style guide. Agree on one voice. Decide on who should have the final say over edits. Know the requirements from your desired publisher, or journal. And decide on who should format everything. It will be more consistent if only one person does that part too.
To use the wiki matrix, start with the choice wizard, then refine your choice based on special features you might need, like MathML, file uploading, and/or a markup language that can easily be exported to LaTeX.
To track changes, make the "Recent Changes" page of your wiki your home page, or turn on email notifications assuming you don't mind the deluge of emails. 
Your manuscript may not look that great on a wiki, but that can be a benefit too. There is no point in obsessing over the formatting of the content until the content has mostly been finalized. 

