It all varies. There can be one event, or several - it depends how well the sorority has gotten to know the girls previously. There are usually more girls invited than there are bids open, unless the sorority is 99.99999% sure the girls they invite will be signing their bids (i.e. if they're really good friends of the whole sisterhood who just didn't rush for whatever reason) and they just have to have attended a recruitment event for whatever bylaw-ish reason.
As to how they decide, that's confidential membership selection info.
