Groups are a new feature available within professional and enterprise plans - as used to facilitate configurable access control (See "What is configurable access control?").
See "How do I upgrade from a free plan?" if you need to upgrade.
Account architects can delete a group.
To delete a group, as an architect, click first on the "Members > Manage groups" option in the top menu.
From the "Manage groups" page, click the "Remove" link underneath the group avatar you seek to delete.
You will be presented with a check question. Just click the tick icon to permanently remove the group from your account.