Sensors and gateways can be added to a particular Group from the Group Settings page under the Sensors tab or Gateways Tab. To access this page, simply click "Settings" and then select "Sensors" or "Gateways." Then, simply click the "Activate Devices" Button to add devices. See below for a screenshot of this setting:
From there, simply put in the MAC address of the devices that you'd like to activate and then click "Activate Samsara Hardware." The MAC address is a unique identifier that is represented by a 12 digit hexadecimal number with colons separating each two numbers that can be found on any Samsara device or on the box that the device was shipped in.
Administrators can also activate new devices by simply clicking on the group name and scrolling down to "Activate Devices." From there, just select the group to add the devices to and then enter the MAC addresses for each device.