For philanthropy events, every member of my chapter has to attend at least two Panhellenic or IFC philanthropy events in the fall to be allowed to go to semi-formal, and two in the spring to be allowed to go to formal. Occasionally, non-PHC/IFC events can count towards these two, but it depends on what they are. There aren't points associated with these events, but the threat of not being able to attend a formal event works wonders in getting people involved
