This is how it works I'm afraid. Salaries are set by negotiation, and market rates. This is usually the reason why employers want salaries kept secret.
I had a team a couple of years ago, 3 senior developers doing basically the same role, one made nearly 50% more than the lowest paid.
Now you may say that you didn't negotiate, as you and your friend were both working internships on the same team, you say it was later on they got the better offer, so this is likely down to changes in the market.
As to what you can do, probably not much. You may have signed a contract as you say you have accepted, renegotiating now means breaking that. If not you could request to renegotiate, but there are risks, you could either cause resentment (which may lead to problems later on), or worst case could mean your job offer was rescinded (as you want more than they are willing to pay), so the question is are you prepared to miss out to try for more? How much of a difference between you and your friend(s), unless it's thousands it's probably not worth taking the risk.
As regards what to say, there's not much to say other than asking to speak to your boss and asking to renegotiate the permanent role. You need to do this before you transition, and the best words to back you are are the pieces of evidence that justify more money, you need to demonstrate why you are worth more (which may seem strange given your friends got offered more, but you'll need to justify), merely stating that someone else got more and you want the same only works when you're seven.
If you don't renegotiate, use the time once you start as a real employee to impress, and work to getting a raise at end-of-year.
It can seem annoying, but think like this: If someone sold you a house for a price, they couldn't come back to you after the deal is completed and say "oh the house is now worth x more, I want more money", timing is always a factor.