[dekeguy]

My Dad tells me that back in the 60's Social Affiliates were fairly common among fraternities and while less so among sororities it was not unusual.
Social affiliates were never allowed to attend ritual or private meetings that were considered confidential but were welcome at any other functions. There were normally two types of social affiliate. One was a student at a nearby university that did not have a greek system or did not have a chapter of the fraternity that the SA wanted. The un-affiliated SA would pay a sort of reduced dues like a pro-rata for parties and events and was welcome at just about anything except as mentioned above. Sometimes the un-affiliated SA would be initiated right after graduation if he had been considered a good supporter and participent. While he was not a brother he was seen as a sort of "first cousin" and could become a full brother if the chapter wanted him and International gave its blessing for initiation after graduation.
The other type was more like what you describe. Someone who was Greek but had no chapter either at their current university or any nearby university. This was a more informal thing and the affiliated SA could be given a standing invitation to hang at the house and attend parties and such. This type of SA paid no pro-rata dues but could kick in to feed the kitty for any function's expenses.
Dad said that he got the impression that among sororities it was often, but not always, a deal where girls who had known each other in High School or had some other connection would be be invited by their old friends to visit and after a few such visits if the rest of the chapter was comfortable with the visitor she would be encouraged to be an affiliate. Not unlike a COB process but not leading to initiation.
In any case, the SA status was not the norm and the huge majority of members entered under the usual procedure.