First of all, you probably should have mentioned your expected salary earlier, before they offered you a position, considering the salary was probably mentioned in the job offer.
The best moment for that is either during the interview or when the company is about to make a job offer.
A polite way now would be "I would like to thank you for the job offer, I'm really interested in the position you offered, but currently I'm paid X currency, are you willing to negotiate ?"
EDIT : You are in a bit harder situation than I thought, if they do a grouped hiring with a fixed salary for everyone, it will be hard to say "I want to be paid more" while having the same work and responsibilities than the rest of the group.
If you are more experienced than the job needs, it might be a better solution to look for another job.
As for this offer, the best strategy is probably to prove you are the one pulling up the team and negotiate a salary raise at a convenient time.
EDIT 2: If you try to have a job not directly in your skill set with a team potentially more trained to those skills and try to be paid more, that's a dead end. You should either accept that you'll have a lower pay for now or start looking for a job that better fits your skills.
Then again you can always try to negotiate with the argument that you were better paid in your previous job, but the risk is that as you will be working with new skills to develop it might get you into trouble if they start thinking you asked for a better salary without the skills that go with it.