Honestly, this would be hard to implement just because of how subjective it is. If you made it simple, like, messaging a newbie to help them learn, people would simply just send a single message, and it'd likely be completely unhelpful. If you make it something like, joining the AANC, then we'll get an influx of unqualified players applying to be mentors for players that actually need mentors.
While an achievement for helping new players would be great incentive, we hope that the want to help new players learn and love the game itself would also be incentive enough! I fear putting an achievement onto this would only invite people who are simply achievement hunters to clog up the process.