The group of friends came to an agreement about the movie they would see together.
Explanation:
Indeed, it skips most of the preceding phrasing while retaining the sentence's meaning and breadth. It conveys the idea that they got to this arrangement jointly and that it was about which film to watch.
A possible alternative:
The group of friends came to a consensual agreement to see a movie together.
This is the greatest answer (not including C.) since the term "consensual" characterizes the agreement they reached, which was mutual and in which everyone participated and accepted the choice. Furthermore, the choice of words and their placement (first the subject, then the verb, and finally the complement) makes the phrase shorter and simpler to understand than the other possibilities.
