Almost all the people in my hall drink and sometimes I sit with them (maybe because they have assured me that they won't force me to drink). Now, my question is: Am I also doing a sin by sitting with them?

There is no doubt that the Qur’ān leaves no room for taking intoxicants. However, the question of socializing with those who violate this injunction has not been discussed in the Qur’ān as such. From the overall picture of Islam, it seems prudent to avoid such company. Yet, there may be situations when it might be against the norms of ethics to cut one's self off from one's society. In an environment where a vice has become pervasive, it is sometimes permissible to regard an evil despicable only in one's heart while fulfilling one's responsibility towards one's society. However, one thing which you should continue doing is inviting other people, that is your friends, to consider your points of view seriously. This invitation must be made with love, understanding and wisdom and never out of hate.