Entering addresses to show on the map

Using location, gmap & views you can.

With location you can use allow users to enter address', you can also allow them to select a point on a gmap or you can set up geocoding in the location module to translate the address into the lon/lat coords.

Then you can create a gmap view in views that display the markers (either on nodes or user depending on what you want to do).

There is lot's of documentation around to set this sort of thing up. Just google it.