If you are in the group details, which can be accessed by clicking on the group name in the left hand menu panel.

You may now go to the group settings, which are located in the upper right of the screen, find the change visibility option in the list and click on the option you desire.

No one in the team except the group members can see this group.

Anyone in the team can see and join this group freely.

Anyone in the team can see the group but only approved people can join.