Please making sure you are using the latest released version of iOS, the latest version of the Philips Hue app and the latest software version of the Philips Hue HomeKit bridge. Start the sharing process from inside the Philips Hue App app > Settings > Siri voice control screens and be sure to enter the correct iCloud account of the user you'd like to share to. Note it can take up to 30 minutes for sharing to work.
If you would like the shared user to be able to create scenes then you will additionally need to set up a HomeKit home hub.
For more information, visit: https://support.apple.com/en-gb/ht207057