Greek houses are not the only group you can join on the campus of "The Sims 2 University." The Secret Society is always looking for new members. However, it is not always apparent who those members are.
Joining the Secret Society
To become a member of the Secret Society a Sim needs to make friends with 3 current members. Members can be seen wearing Llama jackets on community lots. They will not wear their uniform at college residences.

After making friends with 3 members, your Sim will be handcuffed and taken away by limo to the Secret Society.

The Secret Society Building
Each campus has a different Secret Society. They offer generally they same things: a place to socialize with other members, use career reward objects, and a quiet place to study. To visit Sims need to call for a limo by using a phone. Time continues to pass while at the Secret Society. Sims may need to leave to go to class during their visit.