The act of meeting has already happened so it should be expressed in past tense. You use -(으)ㄴ 김에 in past tense sentences and -는 김에 in present and future tense sentences. You can use -(으)ㄴ/는 김에 when talking about something (usually not originally planned) that you do while you are doing something else, because it’s convenient, effective, or easy that way. '우리 만난 김에 식사나 하자' makes sense because the speaker wants to eat together as they met even if he/she didn't plan it before they met. Hope it helps and please follow me at koreanclass.canada on Instagram!