As far as people saying so and so isn't a good XYZ...just combat that by saying something positive. "Oh, she really helped me a lot with the dance-a-thon last month" or "I was having a problem in one of my classes and she gave me all her old notes." People will feel stupid saying negative things if they continually get positive responses. Everyone has a different perception of what a good member is, we just have to recognize that and accept some people will be the social butterflies...some people will be the shoulder to cry on at 3 AM...etc.
As far as the guy thing, ugh. It all depends how small your campus is and how insular the Greeks are about dating each other. In my case, it was very and very!! We had a lot of inter-dating, which if you think about it kind of is understandable...if you all have a lot in common with each other, isn't it understandable that you would be attracted to the same guys? We just decided to be little 60's hippie children about it and say "What the hell, it's all in the family." And you KNOW the guys are going to be on their toes cause they know there are people around to tell stories on them!
There's a difference though, between it just naturally happening and girls pursuing men they have no interest in just to tick someone off. If there's a sister that keeps doing that, someone needs to talk to her and get to the real root of her problem.