Crabgrass

Crabgrass is an online tool which enables group collaboration. It allows groups to do things like share files, track projects, make decisions and store documents online. Using Crabgrass, multiple groups can work together on projects in a way that supports a democratic process. It is possible to rate content, add keywords, and comment on or annotate content. It includes tools like private wikis, task lists, file repositories, approval voting and ranked voting.

There are other similar tools currently being developed which are also designed to fulfill the needs of groups doing info-activism; these include Kune and Lorea.

Collaborating on a planning text or research which needs to be kept private
Keeping in touch, making decisions and giving feedback on the work of collectives in your wider network

Secure and well-tested system
Lots of rights advocates in some areas are already using it

Some people don't find it intuitive to use
The design of public pages for a group or project is a bit basic

Online
FOSS
Easy

https://we.riseup.net/crabgrass

Google Docs, Wikis

Collaborative