Hi @pasik,
if a user has a mobile device configured to sync his emails and calendar, then grommunio-sync will create a tentative appointment.
If the user only uses grommunio-web, then it's necessary to open the meeting request email, and then grommunio-web will create a tentative appointment.
I'm not sure how the other clients, like Outlook, Thunderbird, Evolution etc. handle this.
We have the autoprocessing of meeting requests in our backlog, but there's no ETA yet, when this feature will be available.