You want to enable the setgid bit on the directory containing the files.
See: Wikipedia: Filesystem Permissions
1. Navigate to the parent
of the directory containing the files (e.g. if /var/ftp/userfiles contains the files, navigate to /var/ftp)
2. Change the ownership of the directory containing the files to group 'share.'
root@localhost# chown :share userfiles
3. Apply the setgid bit:
root@localhost# chmod g+s userfiles
From that point on, any files created in userfiles will automatically belong to the share group. Further, any subdirectories created will also belong to group share and
will also inherit the setgid bit.
Note: you will need to manually correct any pre-existing files or directories at the time you set the setgid bit.
Note2: you will also, of course, need to make sure that you and your friend belong to the share group.