The Notes feature is available to all groups and can be accessed by logging into a site and navigating to a specific activity. Near the bottom of the Activity is a small section titled "Notes," and looks similar to this:
Groups use Notes for a variety of purposes. It's a way to leave notes about how the activity went, or things that went into planning it so that in the future you can look back and remember all the details.
If your group is part of the Symphony network, Notes is also a great way to communicate back and forth with your national association liaison. Things like needing promotional materials for a watch party or an email to go out from the main office can be transmitted via a Note here rather than an email - making it easier to keep track of the conversation.
Make sure and select the Update button in the post after you're written a Note. If you enter multiple notes, or if an administrator is messaging you back about the event, you'll see these notes presented in a stream view, similar to: