Merge
A merge is someone who seeks to go with the flow, usually sacrificing ones individuality and integrity in doing so. When someone has a stance on a controversial issue but is silent when around a group with opposing views, that person is being a merge. Also applies to non-serious and humorous situations such as wearing the same outfit as someone else, even if on accident.

One who does not think for themselves.

Effectiveness of the term "merge" is increased when proceeded by a expletive.
Thomas was being such a merge at lunch yesterday.

Yeah, I know. Thomas acts like such a merge around Dave - he just wants Dave to like him.

Stop being such a damn merge and speak your mind!