After updating to Fedora 28, adding a user to a group fails with:
(Wed Jan 2 00:33:14:341408 2019) [sss_cache] [confdb_get_domains] (0x0010): No domains configured, fatal error! Could not open available domains usermod: sss_cache exited with status 2 usermod: Failed to flush the sssd cache.
As suggested, the issue is due to sssd not being configured. If you are not using sssd for authentication, simply remove it and try adding the group again.
sudo dnf remove sssd sudo usermod -aG dialout frank