EasySIMBL is modified "SIMBL" for OS X 10.7, 10.8, 10.9 or 10.10 supporting sandbox, resume. No installer.
GitterのPricing発表に
Additionally, every organisation on GitHub will get one free private org room as part of the free plan.
ってあるのを見つけた。
https://gitter.im/gitterHQ/gitter で質問したら、
For public rooms, chat history is unlimited, but for your ORG room (your one free private room) you will get 2 weeks history.
という回答をもらいました。