You want to use the following
find / -name adduser
If the file is in you path then you can also use
One thing that caught me out for a while was the shell expanding wild cards on me, for example when I tried something like
the search would fail (almost before it got started). The shell was trying to expand the *user in the same way it would if you issued 'rm *user', what you need to use in such situations is
find / -name \*user
to stop the shell being 'helpful'.
Man pages are your friends...