Creating system users and system groups

[Till Kamppeter]

Hi,

I want to snap CUPS and for that also to snap avahi-daemon.

A problem I am running into is that these services use special system 
users and groups. avahi-daemon uses the user "avahi" and CUPS uses the 
user "lp", and the groups "lp" and "lpadmin".

How can I make my snap create such users and groups on install and 
remove them on uninstall?

    Till