I never got interested in advancing the level of my Greek Houses in University, mostly because I didn’t really know what it involved or what the reward would be. Also I concentrate on doing well academically and not the Greek life. Below is a good link that explains how to join an existing Greek House, how to start a new House, the different levels, and information about getting Pledges (new members). It sounds like inviting a sim to be a pledge would be neat. If they accept (have a high enough relationship), you get to influence them for 14 hours without spending your influence points until they become a ful member. I guess the more friends the house has, the higher the level, which then means they can have more members. I haven’t had more than 5 members or so for one house. I guess the members don’t have to live in the Greek House, but then do their friends count towards the House friend total?
http://www.sims2wiki.info/wiki.php?title=Game_Help:Greek_Houses