I'm wondering how to call the phenomenon that happens when people spend a lot of time together and end up sharing similar vocabulary or reusing the other's expressions? Is there a term for this?
Is there a term to define the influence and sharing of common expressions among people?
terminology
You might be referring to mimicry or linguistic mimicry. Mimicry is more broad than just vocabulary and expressions, but the general principle of imitating what others in a social group are doing applies.