I think the assigning an existing Office 365 Group a Team is a bit confusing. I have gone into the People App and I can see my Office 365 groups ( which I am the owner). There is no UI for creating a Team ;-( Whilst I am in People, I can click on the Create '+' icon and I have Standard, Project, Organisational and Interest Groups to create. Nothing to indicate the new Teams (sites). Also, I would have thought I would have PowerShell support for teams as we do currrently for managing Office 365 Groups.
Update
As well as enabling Teams via https://portal.office.com/adminportal/home#/Settings/ServicesAndAddIns , apparently we navigate to http://teams.microsoft.com and start creating. You can also download the windows 10 Teams app , Android App and, or iTunes App. I might be me but Teams is not really using your tenant other than for authentication . Will need some governance and postioning in relation to Office 365 Group and modern team site. #willconfuseusers