Please note that if you get an error su: Sorry for normal user account. Then following workaround needed to get rid of this problem:
1) For security, reason FreeBSD only allows su to user if user is member of wheel group. Wheel group is a special group for administration purpose. Add your normal user to this group using pw command using following:
# pw user mod -G wheel

2) So to add user test to group wheel run command as follows:
# pw user mod test -G wheel
# groups test
test wheel

3) Now su will work for test user.

You can disable this behavior complete for all users (not recommended until and unless you trust ALL your users):
1) Open pam configuration file for su using text editor:
# vi /etc/pam.d/su

2) Look for following line and comment it out:
Line:
auth requisite pam_wheel.so no_warn auth_as_self noroot_ok exempt_if_empty
Replace with:
#auth requisite pam_wheel.so no_warn auth_as_self noroot_ok exempt_if_empty

3) Now all users can use su if then knew the root password.