An update on SharePoint Online is currently being deployed to allow you to connect existing SharePoint Online sites as Office 365 Groups.
This will allow your end-users to have their SharePoint Online available in their Outlook.
When you connect to your SharePoint Online site, open the Gear menu and choose Connect to new Office 365 Groups, then a request to confirm the connection and explanation of what is going on is displayed
When you confirm the SharePoint Online connection as Office 365 Groups you can update the name of the Office 365 Groups name
Within few minute, the SharePoint Online site should now automatically being displayed in Outlook client as Office 365 Groups
NOTE please note the Office 365 Groups connection may failed if you have naming convention in place
If this happens just click retry and update the name of the Office 365 Groups. Unfortunately the current deployment does not provide any information or automatically update the Office 365 Groups name to match the naming convention.