git blame coffee-mode.el --porcelain | grep "^author "|sort|uniq -c
This shows the following authors with more than 15 lines of code:
Chris Wanstrath
Danny McClanahan
Denis Washington
Justine Tunney
Petri Lehtinen
Reuben Thomas (me!)
Syohei Yoshida
Tim McGilchrist
Of course, this is not a rigorous assessment, as it could include commits where the committer has simply reformatted code. I then skimmed through the entire history and did not find any major reformatting commits, so it seems that the above list is indeed correct.
At this stage, given that coffee-mode is unlikely to be that popular, I think it's probably a waste of effort to try to get signatures from any authors who haven't yet signed; so if there are authors who haven't, then NonGNU ELPA is the right place to put it.