Ok, the best option is to reinstall Nextcloud and see if that helps. Existing users added to Users app should be able to login missing ones you just add with the same login name.
After reinstall you need to let Nextcloud know about existing files:
snap run nextcloud.occ files:scan --all
Also you can create a backup before reinstalling.
Also Users app supports groups so you can create one and it should be visible in Nextcloud.