In order to have multiple people able to update the same directories/files:
1. Create directory that multiple user to access.
# mkdir /var/www/html/site1.com
2. Create the group that needs to be able to write to the directory.
# groupadd webdev
3. Change the group associated with the directory.
# chgrp webdev /var/www/html/site1.com
4. Make the directory group writable.
# chmod g+w /var/www/html/site1.com
5. Change the SGID of directory so that any new files retain the group ownership
# chmod g+s /var/www/html/site1.com
6. Create the new user, adding the additional group(set the home directory if necassary).
# useradd -G webdev -d /var/www/html/site1.com user1
7. Change the umask so that newly uploaded files retain group writable permissions:
# vi /etc/ssh/sshd_config
Update the sftp subsytem line to look like so:
Subsystem sftp /bin/sh -c ‘umask 0002; /usr/libexec/openssh/sftp-server’
For chrooted sftp:
Subsystem sftp /bin/sh -c ‘umask 0002; internal-sftp’
Note: if you are trying to make these changes to a directory structure that already exists, any changes to permissions will need to be done recursively.