This is a jukugo word, which usually means on'yomi readings from the kanji. Since 人 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Who's the foreigner? Well obviously the guy wearing jeans (がいじん)!
Tanaka’s daughter got married to a foreigner.
A drunk foreigner tried to pick a fight with me on the train.
Leg length is quite different between Japanese people and foreigners.