Is it possible to grant several users access to the same files?
Absolutely, you can create two (or more) different users on the same folder, one with full permissions and other with read&upload permissions for example.
Even better, you can create one user with full permissions and within this user’s home folder create other users each having one subfolder as a home, this way first user will act as superuser and he can see folders of other users while others cannot go up to superuser folder. See example:
/filegator/repository/ /filegator/repository/john/ /filegator/repository/maria/
In this case give your superuser access to /filegator/repository/ and he will also see John’s and Maria’s folders.