To add a Microsoft Teams team to an existing Office 365 group, first click Teams > Join or create a team, then click Create a team.
Click the link that says Create a team from an existing Office 365 group. From there, all you have to do is select the group you want and add your team. You'll end up in the General channel for the team you added to your Office 365 group.
Note: The Create a team from an existing Office 365 group link will not be available if the existing group has more than 2500 people or has been previously added to a team.