It seems to just come natural to some people, they can just do it without knowing how they're doing it or any of the theory involved. Me, I can sing a melody just fine but I struggle with harmony. Can it be learned, or is it just a natural talent you either have or you don't???
Yes, if you can sing and hear and replicate correct pitches, then it it simply a matter of some training to learn how to sing in harmony with others. I have two recommendations: 1st find a piano and make up a little five note song. Be sure that you can play it a few times and almost memorize it. Then, practice singing a third above those notes while you play the original tune. Just do it on "la" or "do". Got it? So, you are singing one set of pitches while you play another set a third lower. If you want to play both lines and practice each one, then do that too. This short exercise will be the beginning of your ear-training; you can expand on the pattern and intervals after you mater the one above.
2nd: join a choir and sing alto. Make sure that they put you next to a really strong reader and listen to and follow her. Singing alto for a few months will do wonders for your ability to sing in harmony and it will teach you how o read music which will be another great thing for you.
Best of luck!