There are several things that will refresh who is invited to an event, from Spencer in May 2018:
So it will not be immediate but if an event invite/reminder goes out the new people will get it, also if you use the Newsletter function that will refresh the events and add the new members. If a person asks you can also tell them to select the Globe Icon in the top of the Calendar Window to see all events and not filter by Invited Events, this doesn’t need to be a permanent thing, they can change it back later say after the Newsletter goes out and then they will be invited to all the future events.