Fraternities and sororities
Fraternities and sororities are often exclusive, meaning people who want to join need to be invited to do so. This is done during a process called "rushing". During a rush, people who want to join a chapter will talk to current members. If the member think the person rushing is a good fit for the group, they will get an invitation, called a "bid", to join the chapter. If the person rushing accepts, they become a "pledge".