It's pretty light on "invites"... in general, it seems the modern model tends more towards doing this for the summer/post-draft development camps these days, while training camp/rookie camp whittles the list down significantly. Of course, it's also a reflection of our improved prospect depth that we don't really have room for a long list of invites.
So the main "unknowns" on the rookie camp roster are Nychuk and Armstrong.
I guess for fans of certain overage junior scoring sensations out there, I will point out that Nychuk had a very nice overage WHL season with 71 points on defense a couple of years ago. And is already under contract to Milwaukee and likely to play for Atlanta. But that's what a big scoring junior season gets you (see: Sward, Matier case files).
Armstrong is actually the only true "invite" therefore. He played with Molendyk last year, so I don't doubt they took notice while keeping tabs. He is a big body who had a solid overage season on a good team, 34 goals is decent, and he had a good playoff run. If he's the one guy from the summer camp that they opted to bring to this, then he must have made an impression?