I want to contribute, what can I upload?
You can upload almost whatever you like, as long as it is related to MUGEN : characters, stages, add-ons, utilities or sound files for example.
Do I need a formal permission from the creator?
You usually don't, as long as it is considered fair use. Creators themselves usually re-use code, sounds or graphics made by others, without needing any formal permission, as MUGEN is most of the time considered fair use.