You can add the Teams functionality to an existing Office 365 group if you are an administrator of the group.. 

1. On the Office 365 web page, click the Teams app. 

2. Click Add team, which is on the bottom left.

3. Click Create a team.

3. You will then choose what type of team. In this example, Staff Members was chosen.

4. At the bottom of the Create Team dialog, there is an option to add Teams to an existing Office 365 group. Click on Yes, add Microsoft Teams functionality.

5. You will see a list of groups for which you are an administrator. Click the circle in front of the group to which you wish to add the Teams functionality.