Welcome. What you do is your “locations” are Things in your database. We might call them a Location. A Location holds all of the info you to to do “stuff” related to any Location (such as displaying that Location as a marker on a map).
Having done that, we simply let the user select some Locations to create a list of Locations that we then visualize as points on the map.
For an introduction to selecting a single Thing from a list (in this case, using a repeating group), see the following. This’ll give you a grounding in the core concepts:
Understanding the above, you can probably see how to construct a list of items that a user might select. And then your map just visualizes that list.