Hei, we have a group visa type L and we would like to extend it for 30 days more.

Marta, so you must have a group tour, right? If so, you need to confirm this with local entry and exit management bureau. They can decide whether you can extend your group L or not. If they say Yes, then you need provide the following documents and submit them at least 7 days before your current one expires.
1. passport with 2 blank pages and valid for six months at least
2. temporary accommodation registration form
3. filled application forms together with recent passport photos
4. If you are received by Chinese travel agency, then the agency's business license, an explanation letter from the travel agency with its official stamp is required. It should state clearly your personal information, travel itineraries and when to leave.
5. travel expense guarantee, which is used to prove that you still can pay for the expenses during your stay.