Imus,
Fraternities pretty much give out bids whenever they like. For example, my brother started hanging out with the KA chapter at his school during the summer before his freshman year. He accepted a bid from them before Formal Rush even started in the Fall.
NPC sororities have much stricter rules regarding rush than fraternities do. Not only are sororities not allowed to offer bids to incoming freshmen before rush starts, but they are also not allowed to have contact with the potential rushees during the summer.
Hope this answers your question!
