Sounds like Hyomin had some trouble with the acting gig. JiYeon's tweet sounds more like the result of retrospective pondering. They lead busy lives; but there are tons of people out there working their butts off. They'll push through it.
Shin Min Ah is the female lead, so I'm guessing Hyomin will be either the other girl, or the best friend. Regardless of her role though, just being in a drama with Lee Seung Gi is a big deal. *applause*
You can't blame her for being envious. Out of all the active girl groups out there right now, SNSD is the #1 money-maker, and has an enormous fan base. Of course she'd be envious.