There are four main ways you can currently collaborate with your group.
1. Posting on the group wall
You can write posts using the Rich Text Editor window and post it on the group wall. These posts are visible by all members of the group, and are visible to all members of the course if the group is public.
2. Commenting on other posts on the group wall
You can participate in ongoing conversations using the "Comment" button shown next to a conversation thread.
3. Sharing documents with your group
You may share documents on your group's wall via the Rich Text Editor window. Hover and click the picture frame icon, as shown below (in blue when hovered over). You may upload media both from the web and from your computer.
4. Starting a Google Hangout with group members
Clicking on the "Start a Google Hangout" button allows you to do exactly that with the other members in your group.
Note: You may only start a Google Hangout if the teaching team has enabled this function.