The owncloud server at ICM offers a calendar service. You can create your own calendar(s) and share them with other ICM users.
Log into https://owncloud.icm-institute.org
A page opens with a frame on the left. Click on Calendar, then + New Calendar, name it, click Create.
To share the calendar with other users, click the share icon, then enter the names of your coworkers (NB: 20/10/2017, this displays cryptic suggestions. Keep typing the name until there's only one possibility and choose it. This bug has been reported.) Don't forget to tick the "can edit" box so your colleague will be able to add events to the calendar.
That's it. Your colleagues should see the new calendar in their own owncloud calendar page.
Every one of your colleagues with "can edit" rights will be able to add events to this shared calendar.
Install the "lightning" extension: download the plug in and save the file somewhere.
Open it in Thunderbird. Menu on the right > Add-ons, then gear menu > Install Add-on from file...
A new button appears in the top right corner of Thunderbird
Back to the owncloud page where you created the calendar, copy the link in menu "..." next to the calendar then "link". (NOT share public link)
Once you open it, you see a calendar table and a list of your calendars on the left. Right click in it > New calendar... > on the network > CalDav, paste the address you just copied.
Next, choose your ICM address, name the calendar, choose a color and you're done.
This calendar is synchronized between this computer and all of the people it is shared with.
Using DAVx5 app.
url base : https://owncloud.icm-institute.org/remote.php/dav
utilisateur : prenom.nom
mdp : celui de l'ICM
updating password should be no problem.
Follow instructions here: https://aide.lws.fr/base/Autres-services/Messagerie-professionnelle/configurer-calendrier-partage-Android