MapPoint maps are saved as ptm files. These are opened and saved using the File menu:

The MapPoint File Menu
A ptm map file stores all of your data, annotation, and current view information. It does not include the base map information, and requires MapPoint to view it.
MapPoint ptm files are forward compatible, but not backward compatible: A ptm file can be read by later versions of MapPoint, but not earlier versions. For example, a MapPoint 2006 ptm file can be read by MapPoint 2009 and 2010, but not MapPoint 2004.
When you first load a MapPoint file from a previous version of MapPoint, MapPoint will re-locate any data due to possible changes in the road and zipcode database.
Templates
You can also save your MapPoint file as a template. MapPoint template files use the file extension ptt and can be selected from the Save dialog box:

Selecting a template in the Save dialog box
MapPoint templates are very similar to conventional MapPoint files, but you must give a new name when you update and save them. Ie. they are designed to be used as a starting point for new maps – ie. as a template. For example, you could create a template with all of your company’s buildings and locations. This can then be used to create monthly sales reports without having to add the building locations every time.
MapPoint starts all new maps by loading a specific template. Therefore you can change the default map by simply changing this template.

When imported and saved as a template, will custom symbols transfer to other independant users of MapPoint or will importing of those symbols be required on a second MapPoint user?
Yes you can save them as a template – this is an excellent way of only loading them once for yourself. Just re-load the template to start a new map file (and then save the file to a new name of course!). The template can then be transferred to other people with the save version of MapPoint.
When creating pins, 16×16 is the std of larger existing pins. What are the approxiamte sizes of the existing medium & smaller size options? Also, is it possible to change the color of custom pins in MapPoint or best completed prior to upload into MapPoint?