I have the user in group cups and wheel and lp, I made a user tree called pring and enabled cupsd
$ 66-enable -t print cupsd
66-enable: fatal: invalid service file: /etc/66/service/cupsd
I don't print often, actually rare, but the question is why can't I do this as user? It doesn't seem as it would be a cups configuration as the rejection seems to come from 66 not cups itself. Enabling it as a root service works fine and I can reach cups and print as a user.

PS I just tested this as I was afraid this may be one of those rare occasions that dbus is necessary to run in the bg for cups to work, but it doesn't.
if you use -v3 you will see the reason .. 
66-enable: info: Read service file of : cupsd at: /etc/66/service/
66-enable: info: Parsing cupsd service...
66-enable: warning: unable to use cupsd service: permission denied
66-enable: warning: unable to keep common
66-enable: warning: unable to keep information of: cupsd service
66-enable: fatal: invalid service file: /etc/66/service/cupsd
unable to use cupsd service: permission denied < that's the reason
if you look at the service file declaration you will see 
@ user = ( root )
Only root have the permission to use the service.
Now if you try as normal user 
$ cupsd
you will get 
zsh: permission non accordée: cupsd
So, the daemon need root privileges to be launched

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