Translation:I am a volunteer for a good association.
Yup. According to Lernu's dictionary, charity would be "bonfara fondaĵo" or "neprofita fondaĵo".
Bonfara is from bona (good) + fari (to do), neprofita means non-profit of course, and a fondaĵo is a foundation.
Can "asocio" also mean the association between two things or people? Or is it only for the organized group?