Alpha Epsilon Phi does not allow anyone to wear the letters, other than initiated sisters. New members can wear items that say "AEPhi" or "Alpha Epsilon Phi", but not the Greek letters. So the new members wouldn't be allowed to wear the event t-shirts. Neither should anyone else - although you can't exactly go up to people and say, "Hey, you're not an AEPhi, hand over the shirt!"
I could see a chapter making a big stink after the fact, though; the event planners should contact each group ahead of time, and ask if they can use that group's letters.