I know that it used to be once they signed with an agent (historically as a move to prepare for the draft) they became ineligible. But I have to think that with the current NIL landscape it would be absolutely critical for kids to have someone advising them. I know that it seems like EVERY kid in the portal is getting $400K to play college ball, but the reality is that probably only a small handful of them are getting anything close to that. Most of thousands of kids who entered the portal will end up signing somewhere to play just for a scholarship and a stipend, and I think that a lot of them might not understand that when they enter the portal.
Of course, some agents might not actually help - they could actually encourage kids to jump into the portal when they would be better off staying put, but in a lot of situations, they could help.