nagios create readonly user

# cd /etc/nagios/

htpasswd passowrdfile readonlyuser
# htpasswd htpasswd.users nagiosuser

Enter you password.

Then edit cgi.cfg file.There are different access you can enable.
# SYSTEM/PROCESS INFORMATION ACCESS
# CONFIGURATION INFORMATION ACCESS
# SYSTEM/PROCESS COMMAND ACCESS
# GLOBAL HOST/SERVICE VIEW ACCESS
# GLOBAL HOST/SERVICE COMMAND ACCESS

"GLOBAL HOST/SERVICE VIEW ACCESS" access is enough for read only users.
authorized_for_all_services=nagiosadmin,nagiosuser
authorized_for_all_hosts=nagiosadmin,nagiosuser