hmm, I have no idea how city-wide chapters work. I can assume that people who are trying to establish one would have to somehow advertise through the city/area. Like I know a lot of the NPHC groups have city wide chapters and they are not affiliated w/ any schools...well I don't think they are. To me, when I hear city-wide, I think strictly the city and that anyone who meets the criteria for the sorority that wants to start a chapter in that city/area can apply for membership.
Now on how to advertise? Wow that's a big task. I guess flyers? Maybe the newspapar would be a good way. Probably word of mouth too. If you could somehow advertise on websites that are used often like Ebay, or any search engines, etc, that could help. It seems like it would be hard. But you know, if it's city wide, you could probably advertise at the local colleges/uni's. I'm not sure if city wide chapters only take graduates or both, but you can still advertise at the schools in the area. Hope this helps!
I really have no clue about this. I'm just guessing. You might want to ask members of sororities/fraternities that have city-wide chapters how they advertise.