In seventhings, field groups are the tiles in which the object fields are arranged in the detail view of the object. For more information, see this article: What are groups?
To create a new field group, open an existing object field (or create a new one), click in the group field and type the new field group name into the text box.
The new field group is now created and you can start assigning your object fields to it.
Note:
To delete field groups, simply remove all assigned object fields from the group.
The group will automatically be deleted as soon as no object field is assigned to it anymore.