The first thing I would note is that joining a frat is a choice. In other words, there is not need to go anywhere near the possibility of a situation you might dread.
Thing two is that fraternities at Dartmouth have a range of reputations. Students tend to gravitate toward the houses that suit their personal and social style. The system is big enough that there is a range of options and these options are pretty clear once you start talking to people. Upperclassmen are pretty open about what is involved with each house, which mostly involves degrees of drinking. In other words, you can have good sense of what lies ahead.
Above all, if you do not like what is going on you can always say no or walk away.