I think this is dependent on the formatting and structure of the invitation; the invitations sent to me by Outlook users, and by other Thunderbird users, for instance, add themselves to the Lightning calendar. I don't know of any settings in Thunderbird or Lightning that affect this. In the cases where I get an email message that includes a written-out invitation or notification (as opposed to one that is calendar-generated) I use right-click and "convert to event". This copy-and-pastes the body text of the message into a new Calendar event, leaving me to fill in the time and date.