You can post a message to get stuff discussed with your team, to share files, or even make a decision.
From any Activity page, click on Write your message... and type what you want to say.
Check the Share on information to make sure you're posting to the appropriate group of people. You can share a post with members of one or more Subjects.
When you're done, click on the Share button.
Your teammates get notified inside the application and by email and on their mobile according to their personal notifications settings.
In a message, you can paste a link, attach documents from your computer or cloud accounts (Google Drive, Dropbox, Box, OneDrive or Evernote), and even add a poll.