This will tweak the HTML on Facebook groups so that hitting Enter in a comment box no longer submits the comment. It adds a Submit button a la the non-groups comment UI.
Tested on Chrome (by me) and under GreaseMonkey in Firefox.
It is a total hack and I make no claims about its elegance or the likelihood that it will continue to work over time. Send me a patch if it's deficient in some way; not really interested in bug reports since it's something I wrote on a lark and just put online in case others wanted it.
Development version (this will probably crash your computer and make all your hair fall out)