How can I share the SSH keyes between two users on a CentOs 7 machine?

First and foremost: use ls -lhAZ (-Z on a SELinux enabled System), it shows Linux file ownership, permissions and SELinux label.

Reset permissions on the directory and file:

# DAC - Restore permissions
chmod 0700 ~anobili/.ssh
chmod 0600 ~anobili/.ssh/authorized_keys
# Restore ownership
chown -R anobili ~anobili/.ssh
# MAC - Restore SELinux 
restorecon -R ~anobili/.ssh

This is what it's supposed to look like:

$ ls -lhaZ | grep .ssh
drwx------. 2 user group unconfined_u:object_r:ssh_home_t:s0  4.0K Feb 21 18:52 .ssh
$ ls -lhaZ .ssh
-rw-------. 1 user group unconfined_u:object_r:ssh_home_t:s0 0 Feb 21 18:53 authorized_keys