I have a CSV file in which each row represents a person and includes a location for them, which I want to map in QGIS as point layer. However, I would like some people to appear as a point in multiple locations simultaneously. What would be the best way to format my spreadsheet in order to reflect that in QGIS?
Simplest way - create a new row for each of their locations.
A possibly better way depending on your application, source of data, etc. would be to use well-known text (WKT) format. You would want to create a MultiPoint object. Formatting is listed at the top of the second table on this Wikipedia page. How easy it is for you to get the data into this format depends on how your data is organized before it gets to your csv file. I've successfully used MS Access queries and Excel functions to create WKT fields in similar situations. A lot of how complicated it gets will depend on whether you have a consistent number of "sightings" for each person. If they vary, you may need to do some scripting to create the multipoint string.