Customizing users and groups on a systemd-based (QEMU) system:

INIT_MANAGER = "systemd"

and making use of rootfs-postcommands.

# Handle inherits of any of the image classes we need
IMGCLASSES = "rootfs_${IMAGE_PKGTYPE} image_types ${IMAGE_CLASSES}"
# Only Linux SDKs support populate_sdk_ext, fall back to populate_sdk_base
# in the non-Linux SDK_OS case, such as mingw32
IMGCLASSES += "${@['populate_sdk_base', 'populate_sdk_ext']['linux' in d.getVar("SDK_OS")]}"
IMGCLASSES += "${@bb.utils.contains_any('IMAGE_FSTYPES', 'live iso hddimg', 'image-live', '', d)}"
IMGCLASSES += "${@bb.utils.contains('IMAGE_FSTYPES', 'container', 'image-container', '', d)}"
IMGCLASSES += "image_types_wic"
IMGCLASSES += "rootfs-postcommands"
IMGCLASSES += "image-postinst-intercepts"
inherit ${IMGCLASSES}

The end result:

IMGCLASSES="
  rootfs_rpm
  image_types
  qemuboot
  qemuboot
  license_image
  populate_sdk_ext   
  image_types_wic 
  rootfs-postcommands 
  image-postinst-intercepts
"