Running vsftpd on CentOS, a user was unable to upload a file to a folder for which the user had group permissions.
Thanks to this post here, I was able to figure it out.
Needed to edit vsftpd.conf and update to include the following lines:
local_umask=002
file_open_mode=0777
anon_upload_enable=YES
anon_mkdir_write_enable=YES
After restarting vsftpd and making sure the relevant folders are group-writable, it works!