If you’re talking about watching:
It varies by school. Some schools allow no one, not even family or agents. Some schools have made it completely open. Some schools will verify you are agent for player participating and will let you watch from stands. It all just depends.
If you’re talking about attending/participating:
There are rules that have to met to be able to participate. If you meet these NFL rules you can then find out if the college allows players from outside the university to participate. Most do not.