It is not totally unheard of to have one name be used for different organizations. A quick gander at
http://www.greekpages.com will show you that
. I hear that even my own sorority's name is being somewhere else (although they are a very small group....and have not registered there name or anything like that).
Legally speaking, a [corporate] name can be trademarked. There are different levels of trademarking; state, federal and international. I could not tell you the fees off the top of head....but it's usually not cheap. At least $200. If you incorporate the sorority, you will need to make sure the name is unique - but only among other incorporated entities in that state. It is unlikely that this other group is incorporated in your state as well. If it is, you could incorporate in another state (check laws though...sometimes board members of the corporation must be state residents).