How sure are you that your friends know what they are talking about?
If you want to do something, you've two choices -
1) You negotiate for more before you sign the papers. It will be difficult to do this if you have already verbally accepted what they offer. Do you have any reasons which you can give for asking for more? Any other offers out there (or which you can convincingly say are out there?), or additional costs which you can factor in? Even if not, you could say "Since you made the offer I've researched the market in this area, your offer is below what would be expected, are you able to offer me X?"
or
2) You accept the offer, start the job, and then start working on asking for a rise, based on what you have shown you can offer, the value you provide to the company, and the costs to the company if you leave. You could ask for a rise after 6 months, maybe, depending on how things are going. Are you going to be happy working those 6 months at less than you now think (based on what your friends say) you are worth? Are you likely to get the rise in 6 months time?
Unless you are desperate for a particular job, it is a good idea, when you are made an initial offer, to ask whether they are able to make a better offer. If you don't ask, you don't get.
It's also good to negotiate from a position of strength - which means that you feel you have other options if the negotiations don't work out. Always start a negotiation knowing that you have a plan of action whichever way it turns out.
Also, be aware, this is an anonymous internet post, as are nearly all the others on this site, and you follow any advice in it at your own risk.