Adding a Microsoft Teams instance to existing Office 365 Groups directly from SharePoint Online team sites is a new Office 365 feature. Only users who are group owners can perform this action, and the option only appears if Teams is enabled in your tenant for SharePoint team sites already connected to an Office 365 group, but not yet connected to Microsoft Teams.
With the availability of this feature, Office 365 group owners will be able to add a Microsoft Teams instance to existing Office 365 Groups directly from SharePoint Online team sites. This provides a direct, one-click mechanism to create a Team and associate it with an existing group from the bottom-left of the home page of a SharePoint Online team site. This complements and simplifies, the existing multi-step capability of doing the same from within the Teams experience.
Microsoft will begin gradually rolling this out to Targeted Release organizations in late October 2018. The broader rollout is to take place over the following weeks, and full global availability before the end of this calendar year.