Who can participate in a mapping of the garbage?

A mapper could be anyone who is old enough to evaluate what is garbage and how much of it is there.

You could either use a skilled group of people or work with large group of volunteers. You could either use independent mappers who work on their own or you could organize public mapping events-weekends where certain areas will be divided between volunteers and get covered very quickly. It is possible to mix those approaches as each of them have their advantages. 

Mappers are usually people who orientate very well, have a good memory and could (even without GPS) determine the coordinates of a trash point and could communicate with people. Communication with other mappers in their area or with local people in his/her area helps a lot.

Other people can help mappers as well with adding mapped data to the database or just gathering together the information of their local territory/region or the data of certain group of mappers. The person entering information to the database doesn't definitely have to do the mapping itself. This way you can combine skilled mappers with skilled data-inserters.

The ideal mapping team has a 4WD car, 2 people, a GPS or GPS-equipped mobile phone (with loaded batteries!), and a camera. If there are no major problems, such a team was able to map ca 200 square km per day.