Dime con quien andas, y te dire quien eres.[*]
Tell me with whom stand, and you tell who are
Tell me who you are with, and I will tell you who you are.
Jorge learned this proverb from his mother back when he lived in El Salvador. According to him, you will most often hear this being said by older people to younger people. He compared its use with the American If your friend jumps off the bridge, will you jump too? saying that a Salvadorean parent might use this proverb instead. Although the two proverbs are slightly different in meaning, they can both apply for young, impressionable people who are often easily influenced by their peers. For him, the proverb is a warning to heed who you hang out with, as you will become like them. If you hang out with smart people, you will get smart too, he says, but if you hang out with dumbasses, well he chuckles. He added that it can also at times mean, that you can tell what someone is like by observing who he hangs out with.
I agree with Jorge that the proverb can be about the way people can take on the characteristics of those they are most often with, although I would tend to think it speaks more directly about the way similar people tend to congregate. I am also unsure how much I agree with the comparison with the Anglo proverb. I might suggest instead that it is more similar to birds of a feather flock together.
[*] Annotation: Santiago, E. (2006). When I was Puerto Rican. New York: Da Capo Press, p. 243