This thread is jarring some memories that I stuck way back in my brain but only now (due to the jarring) are coming forth. Will share son's experience with similar camp scenario:
Son got camp email from D1 school summer of rising Jr. year. Don't remember the wording but it seemed to suggest it was an "invite only" camp, blah-blah-blah. I was suspect for all the same reasons others have mentioned above. Not this dad's first rodeo nor first $300 spent on a camp at that point.
The way it passed muster for me however was the fact that the camp did not appear to be advertised at all. There was no website for the camp, no mention of it on their normal baseball camp website that had advertising for other camps later that fall, but there was no mention of this camp that RC had just invited son to saying/suggesting the invite only nature. The RC's email was not personal at all. Looked like a regular camp invite with the invite only wording, or something to that effect. The fact that I could find no advertising for it was enough of a litmus test for me to believe it would be a legit invite only setting.
Sure enough, there were only 32+- kids at the camp. All good looking ball players. HC reiterated the invite only status of the camp and thanked all for coming. Skills and drills, with facilities tour in AM, with a game in afternoon of 16 vs 16, with many POs. So lots of reps and exposure for everyone.