What are the most popular student activities/groups?
A lot of people play sports (whether competitively or club) and a lot of people also participate in a cappella groups, which are huge on campus. People are also very involved in campus publications such as the Olio (yearbook), Amherst Student (newspaper), the Indicator, and others.
During freshman year, you'll tend to be friends with the people in your own dorm. The best time to meet people in other dorms is during orientation week. Honestly, you don't want to associate exclusively with people in your dorm. Branching out is very helpful and refreshing so that you always have someone you can talk to in the dining hall.
Social scene is so-so. TAPs (The Amherst Party) are put on by Social Council and they can vary from really good to super crowded and disgusting. Some of the best parties are put on by students orgs, and the music tends to be better. There isn't much pressure to drink, unless you hang out with people who put that pressure on you. It's college, and everyone respects everyone else's decision for the most part.
Most weekends there are parties, movies, performances, etc. to keep you busy. Also, going into town to a restaurant or taking the bus to the Hampshire Mall to watch a movie is a way to get off campus.
There are no frats or sororities on campus.
As far as dating goes, some people are in relationships, whereas a lot of other people just hook up at parties. The common culture doesn't seem to be one of looking for long-term relationships.