I have over the past year(s) sent several requests to fix this:
The CalDav implementation of SM does not work fine to create invites in MacOS Calendar or iOS Calendar (havent tried in other OSs).
When creating an event in Mac Cal or iphone Cal, there is no way to add attendees to the event, and therefore, it does not send the invitations... The only way to do this is by creating the event in the webmail. If the event is created in icloud calendar, the add attendees option is available.
And recently I asked if this is a feature that could be added since many clients ask for it:
Share their calendars with external people (like you do on gmail, icloud, etc.) outside their domain
Please add these to your list in case this is not available yet in v.14. These are very important features that need to be added!