When Plesk creates an account for you it will always assign the httpdocs folder to the "psaserv" group, and all files within the httpdocs folder to the "psacln" group.
Sometimes you may run into trouble should you accidentally modify these groups (especially when you change the group of the httpdocs folder from psaserv) so should you need to restore these groupings you may run the following commands in succession:
chown -R your_plesk_username:psacln /your/httpdocs/directory
chown your_plesk_username:psaserv /your/httpdocs/directory
What the first command does is make sure that all of the files and folders within your httpdocs folder are correctly within the "psacln" group. The negative side effect of using the "-R" directive is that it also changes the permissions of the httpdocs directory itself, leading to the second command which only changes the httpdocs folder itself to be part of the "psaserv" group, while leaving the files and folders within with the correct grouping.