Please note that the radiusd does run as user 'radiusd' and
group 'radiusd' per default after installation.
(/etc/raddb/radiusd.conf)
This was done because of security reasons and is possible with many
authentication types (users, LDAP, SQL ...)

If you would like to use unix (shadow) authentication (Auth-Type = System) 
or PAM (Auth-Type = Pam) you have to change the values to run the daemon
as user/group root:

user = root
group = root