Adding user to ownership in addition to another user

Solution 1:

According to the traditional file permission that's not possible: a file has only a single owner.

Your approach including admin to apache group should work.

However most modern filesystems support ACLs. If you want to use ACLs to give another user read (r) and write (w) permissions then you can use the command setfacl:

setfacl -m user:admin:rw /path/to/webserver/www 

Even if the filesystem you are using supports ACLs, check that it is mounted with support for ACLs. In that case you need to issue the command mount and get the dev related to the filesystem, for example:

[root@centos]#mount

/dev/sda3 on / type ext4 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
tmpfs on /dev/shm type tmpfs (rw)
/dev/sda1 on /boot type ext4 (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)

The dev for the root filesystem is /dev/sda3.

Now with the command tune2fs you have to issue the command:

[root@centos]tune2fs -l /dev/sda3 | grep acl

Default mount options:    user_xattr acl

In this case it has aclenabled. If it wasn't the case then you have to modify /etc/fstab to include aclon the options as it is explained here.