If I recall colonization procedures correctly (it's been a long time, so don't quote me on this)... I believe the girls do the first round of rush in order to introduce their chapter to the rushees and just let them know that DZ is on campus and available to them. However, because the colony is new, they don't have the resources or the experience with formal rush to really compete well with the more established groups on campus.
Rush on the sorority side can be an expensive process, and it's also a rather complicated one if you're not used to dealing with all the ins and outs. Rather than throw the colony into the 'lion's den' immediately, DZ will start out slowly with the help of National consultants who will come down to assist the colony with the selection of girls.
So, what DZ will do is have a first round, which is usually the most informal and least expensive round of parties for a chapter to participate in, and during the round the girls will basically say to the rushees, "Hey, here we are... get to know us a little, and keep us in mind during Rush. We'll be around after the end of Rush if you're interested in pursuing membership." Kind of a "meet and greet and remember us for later" kind of thing.
Hope this helps a little bit!
