All the documentation and guides I found say that to enable a persistent journal on Debian you just need to create /var/log/journal. It is true that once you create that directory you will get a persistent journal.
All the documentation and guides I found say that as long as you are in group adm (or sometimes they say group systemd-journal) it is possible to see all system logs by just typing journalctl, without having to run it as root. Having simply done mkdir /var/log/journal I can tell you that is not the case. All you will see is logs relating to your user.
The missing piece of info is contained in /usr/share/doc/systemd/README.Debian:
Enabling persistent logging in journald
To enable persistent logging, create /var/log/journal and set up proper permissions:
install -d -g systemd-journal /var/log/journal
setfacl -R -nm g:adm:rx,d:g:adm:rx /var/log/journal
-- Tollef Fog Heen <firstname.lastname@example.org>, Wed, 12 Oct 2011 08:43:50 +0200
Without the above you will not have permission to read the /var/log/journal/