He or she can write letters
Or make phone calls. Or create videos to be uploaded to YouTube. Or write books, and self-publish on Amazon. Or....
The "trick" is in using their God-given gifts and talents. Not settling for any particular method or means, but finding the one(s) that work for that particular individual. A sports announcer once said he had a face made for radio; give that some thought.
You've said that you don't usually have the opportunity to disciple others; have you considered helping out at a soup kitchen, or church food pantry? Something that actually puts you in contact with people? (And in such cases, often the same people on a recurring basis, allowing you to develop a relationship and disciple them)
Hands don't necessarily preach; but they can give out food, and help raise up the arms of the preachers. Feet don't necessarily preach; but they can take bibles and the message out to the world. Ears don't necessarily preach; but they can show God's love and be His ears by listening to those that need to believe someone is listening to them. And all of them--hands, feet, ears, mouth--work all to the same purpose.
Don't ask what others are doing, and especially don't assume they're doing nothing. They're doing it the way God designed them, and you should be rejoicing that they are, not saying that because they're not like you, they have lesser value.
If you had truly read and understood 1 Cor 12, I wouldn't have had to post that. But I just read another thread of yours, where you think people should write letters; and where you asked "What are you doing? Nothing!" It's obvious you don't understand 1 Cor 12 at all.