Well which ever one you don't have to be invited to we will be at going to earn an invite to the invitation only one.
Anyone can go to the camps that are held in the summer. You should look into those if you are trying to get your sons recruited by Nebraska.
As a side note, I used to post on a NU message board where a woman had 2 sons playing high school football and they were recruited by D-1 schools. Now, this woman seemed nice, but due to the way she would post updates about the recruitment of her kids, she gathered a following of posters that were getting mad when NU wasn't offering scholarships for her sons. It was just a strange thing to follow. I am sure you are much more grounded, and I am sure that your sons are very talented players. I just hope you don't make things too personal IF your kids don't get scholarship offers by Nebraska.