I've discovered that for journalctl only exist user(s) and others: no
group permissions.

Example: exim4 sets read+write permissions to root for logs, and
read-only for group "adm". If you are member of "adm", you can read
EXIM4 logs.
Depending on distribution, journalctl allows you to read recorded logs
depending only on you being root or not: In some distros (eg. Ubuntu)
any user can read any journal, and (eg. Debian) only root can read
journals (nobody else can read anything else).