1.- Run a Radicale server at calendar.coopdevs.org
2.- Create a lightweight Odoo module that will
.ics attached " to also put the same .ics on Radicalecurl -u 'user:password' -X PUT https://calendar.coopdevs.org/user/calendar --data-binary @invitation.ics
3.- Now you can sync calendar.coopdevs.org/user/calendar in your favourite calendar client.