My pensieve

useradd and groupadd are the posix or SUS-standardized tools to create
users and groups. You can expect them to work (almost) identically on all Linux
distributions as well as on HP-UX, Solaris, AIX, ...

adduser and groupadd are distribution specific wrappers which make it
simpler to create users which conform to some (distribution-specific or
local) policy. The options vary wildly between different distributions
and other UNIXes may not have these commands at all.