How to add text at the end of each line in Vim?

This will do it to every line in the file:

:%s/$/,/

If you want to do a subset of lines instead of the whole file, you can specify them in place of the %.

One way is to do a visual select and then type the :. It will fill in :'<,'> for you, then you type the rest of it (Notice you only need to add s/$/,/)

:'<,'>s/$/,/