Concept
Not yet checked against a primary source — how the pages are sourced
The set of physical disks a controller gathers together to work with. A drive group is a pool, not a layout: it marks off "these disks are mine to arrange", and the arranging, striping, mirroring or parity, happens one layer down.
Forming a drive group is a gathering step. The raid engine takes a set of physical disks under itself, and from that point on it can divide them up as it likes. A drive group by itself has no striping, no mirroring, no parity of its own; it just says which disks belong together. The layout is applied in the span, and what the operating system is finally given is a virtual drive. The one layout a group does carry is the stripe that joins its spans in a nested level: a RAID 10 group is two or more mirror spans, striped together at the group.
The words are a controller vendor's. In Broadcom's MegaRAID documentation "a drive group is a group of physical drives. These drives are managed in partitions that are known as virtual drives", and a drive group carries a RAID level: a RAID 1 drive group is two disks, a RAID 10 drive group is several RAID 1 sets spanned together. Linux md has no separate word for it: the set of devices given to mdadm --create is the group and the array at once. Windows Storage Spaces calls its gathering step a storage pool, from which storage spaces are carved.
Why keep the layer at all, if some systems do not name it? Because the gathering and the layout are two decisions, and they can differ: one group of eight disks can hold two spans of four with different layouts, or one virtual drive that uses part of the group and leaves the rest free. Seeing the pool as its own layer is what makes those cases easy to describe. The figure under virtual drive shows the three layers nested, with such a group as the outer box.