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.
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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.
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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.