It can use when you are talking about relationships between people and people. 〇かのじょと　わたしは　仲がいい。（She and I are really close.) 〇AさんとBさんは　仲がわるい。（A and B don't get along.)

仲 comes from the same source as 「中」. 仲の良い友達。 なかの よい ともだち。 Good friends. 気楽な仲間。 きらくな なかま。 Easy going friend. 犬猿の仲 けんえんの なか。 Like cats and dogs. (仲が悪いということ) (なかが わるいと いうこと)