I have a client who is looking for a solution that I'm not sure exists in Microsoft at this point, but I wanted to check the hive-mind.
Their basic requirements would be, as part of a SPFx web part we already have, an "Add to My Calendar" button. They do NOT want ICS as a solution due to 1) it doesn't update if there are changes to the original event and 2) they reported issues with Mac users and ICS in the past, and the client is at least 50/50 Mac.
Ideally, they'd like the user to be able to access a link that adds them as an attendee to the original meeting so that they can get updates if the event changes and so the event organizer can see who is signed up. Is this a possibility at all or do we need to bring in a third party solution?
You can use an Outlook plug-in like ContactMonkey to help with this issue. They have an event management feature that allows you to create events and input a CTA or link to register. If there are any changes, your attendees will be notified (whether they're a mac or a windows user).