Add user to docker group

Is there a recommended way to a user to the docker group? Through the GUI it does not work and sudo usermod -aG also gives an error.

same error as with usermod: [sss_cache] [confdb_init] (0x0010): Unable to open config database [/var/run/sssd-cache/db/config.ldb]
Could not open available domains
[sss_cache] [confdb_init] (0x0010): Unable to open config database [/var/run/sssd-cache/db/config.ldb]
Could not open available domains