The word “with” can be expressed with 랑 :

친구랑 점심 먹었어요 = I ate lunch with a friend

(친구 = friend. 랑 = with. 점심 = lunch. 먹었어요 = ate)

However 랑 is used only after vowels. 이랑 is used after consonants :

제범이랑 도서관에서 공부했어요 = I studied in the library with Jaebum

(제범 = Jaebum. 이랑 = with. 도서관 = library. 에서 = in. 공부했어요 = studied)

(이) 랑 can be used connect two nouns together and can have the meaning “and” :

수영이랑 농구를 좋아해요 = I like swimming and basketball.

(수영 = swimming. 이랑 = and. 농구 = basketball. 를 = particle. 좋아해요 = like)

(이)랑 can also be used in a question :

누구랑 살아요? = Who do you live with?

(누구 = who. 랑 = with. 살아요 = live)

The word 함께 can be used with (이) 랑 to mean “together with” to emphasize the togetherness:

작년에 가족이랑 함께 모였어요 = Last year, I got together with the family

(작년 = last year. 에 = particle. 가족 = family. 이랑 = with. 함께 = together. 모였어요 = get together)

Also, (이) 랑 is used more in everyday speech, and you could take the sentence above and replace with other connectives like 과, or 하고.

같이 can also be used with (이) 랑 to mean “together with” as a way of emphasizing the togetherness :

남자친구랑 같이 여행갔어요 = I went traveling together with my boyfriend

(남자 = boy. 친구 = friend. 랑 = with.  같이 = together.  여행갔어요 = went traveling)