Yammer is a portal from Microsoft that allows people to communicate in an organization.
To log into Yammer, go to https://www.yammer.com/:
You can also access Yammer from the Office 365 menu:
Logging in, you will see the home page, with posts from different Yammer users in your organization:
Select + to create new groups in Yammer:
These groups can be internal and external:
You can also send private messages to coworkers, post status updates, share documents and many other things.