Both are good questions and I will do my best to answer them for you here. Fraternities at RPI and around the country look for new members in a process called rush. The rush period is a chance for brothers to find new people that might be interested in joining the house and the perfect chance for the non-brother to get to meet a whole bunch of new people. It is an excellent way to meet people, and in fact that is really the whole point of any rush program. The rush period at RPI is at the beginning of each semester.
Most houses rush both semesters, and many people rush multiple semesters before joining a house.
Houses hold events that are generally a lot of fun and invite people to come to them. Most events are open to the general male population of RPI, some are closed events. At the events the brothers get to know you, and you them and many friendships are formed. At the end of the rush period bids are given out to rushees that the brotherhood felt would make good new brothers. A bid is an invitation to join the pledge program for the fraternity so that the process of becoming a brother can begin.