On most distribution
addgroup are interactive 'convenience' wrappers around the commands
You can find addgroup using the command
which addgroup, on my machine (Ubuntu 11.04) this lives in
On my box
addgroup is a perl script that prompts for various options (interactively) before invoking the
groupadd is usually preferable for scripting (say, if you wan't to create users in batch), whereas
addgroup is more user friendly (especially if you are unfamiliar with all the options and flags).
addgroup also takes many options via the command when you invoke it, but it is primarily intended as an interactive script.
Interestingly on my box
addgroup is a symlink to
adduser, the script checks the name it was invoked under and performs different actions accordingly.