Since draft pools and slotting went into effect in 2012, all of 5 first or supplementary/compensatory/competitive balance round picks did not sign and really only one is really a direct comp to Groome, IMO.
2012 - Mark Appel - A junior at Stanford and expected to be overall #1, he fell due to signability concerns and was taken 8th by Pittsburgh. He instead went back for his senior year. Was drafted #1 the following year by Houston. He reportedly turned down a predraft offer of $6M from Houston and then $3.8M at slot from Pitt in 2012 and eventually took $6.35M the following year. Not a very good comp for Groome, as he had leverage, but the argument can certainly be made that perhaps the extra year in college might have hurt him in the long run.
2013 - Phillip Bickford - High school draftee was taken 10th overall by Toronto but turned down $2.9M and enrolled at Cal St Fullerton. After one year, he optioned the Bryce Harper playbook and transferred to the College of Southern Nevada so that he could be draft eligible after two years instead of three. He subsequently was picked 18th by SF and signed for $2.3M. This is the closest comp to Groome, IMO.
2013 - Matt Krook - High school draftee, taken 35th overall by Miami Marlins, failed his physical and Miami withdrew offer. Not very relevant, but should be noted he enrolled at Oregon, blew out his elbow and was subsequently just selected in the fourth round by SF.
2014 - Brady Aiken - Not even going to bother with the details as we all know what happened and it's not even remotely relevant.
2015 - Kyle Funkhouser - Louisville junior draftee, he was taken 35th overall by LAD and turned down a reported offer of $2M (slot being $1.75) to return for his senior year at Louisville, where his stock plummeted and was just drafted in the fourth round by Detroit. He's likely looking at a signing bonus in the $500K range.
Now, again, I really only think Bickford is truly relevant as two of these guys were college juniors with leverage and two were injury cases. Bickford is the closest comp. But I wanted to list them all to illustrate that since the new rules have gone into effect, only 5 of 182 picks taken before the start of the second round have not signed and only one that was in the place Groome is right now.
Guys just don't hold out like they used to anymore because there's a cap on the potential upside to rolling those dice now. Yes, if he went to college or Juco and came back to the draft he could potentially double his money if he ended up going #1 overall, or even if he went top three. But *only* if he went top three. And that's ginormous risk to take. In the previous iteration of the draft, he could go at virtually any slot and play whatever games he and his agent wished. That game can't really be played anymore because the system has changed so much. He can go to Juco for a year but next year is considered a much stronger draft class. So he plays the game again and returns to Juco to enter 2018. And at that point, even if he performs and matures perfectly, who knows what other draftees emerge over the next couple years?
Most of the major draft guys now assume that absent injury complications, every first round pick signs. Law, Callis, Mayo, every time I see them asked about a particular player, they always answer that they expect them to sign. I think there's no way they don't ink this kid.