How do I set up multiple meetings in an Email that allows invitees to choose a meeting to attend

Link for Gmail

To make a link for Gmail, you need a gmail account. In google calendar, create the event with all the details. When it's done, publish it. (3 dot, then publish)

In the pop up, copy the second link and insert it in your email with for example the text "add to gmail" or something like that.

Link for outlook/mac

Create the event in your outlook account ( appointment, not meeting). Open it, click on "file", then "save as", select the format "ICS" and save it on your desktop. drag and drop also the event in your outlook to have a "outlook" item

The hardest part will be to store it somewhere everybody can retrieve it. 

If the event is expected in less than 90 days, you can put those 2 files in your OneDrive, create a "anyone with the link" and copy that link in your email .

The link to the ICS file you can call it "iCAL" and the link to the outlook item, simply "outlook".