Once you've created a group, here's how to actually put devices in it.
There's exactly one place to assign devices to a group: an "Assign Devices" button on that group's own page. Your device list doesn't show groups at all, and a device's own details page only shows which group it's in as read-only text — neither one lets you change it. Assignment always happens from the group's side, not the device's.
This control is only available to the account owner or a team member with access to your whole fleet. If you're a team member scoped to specific groups, you won't see it at all.
Opening a group's assign picker shows every active device on your account, not just the unassigned ones — with tabs to filter to All, Unassigned, or Assigned. Each device's current group is shown right in the list, so if a device is already in a different group, you'll see that before you do anything.
Check a device that's already assigned elsewhere and a note appears right next to its name: "will be moved from [old group] to [new group]." Assigning a device that's already in a group doesn't add it to both — it moves it, and the picker tells you exactly where from and where to before you confirm.
You can move a device from one group to another anytime, but there's currently no control that sets a device back to Unassigned while leaving its group intact. The only way a device becomes unassigned again is if the group it's in gets deleted — which unassigns every device that was in it, not just one. If you want a single device out of a group without deleting the group itself, moving it into a different group is the only option today.