I was surprised by

$ cd /tmp
$ mkdir foo
$ cvs -d /tmp/foo init
cvs [init aborted]: init to an existing repository is restricted to members of the group cvsadmin
$ grep cvsadmin /etc/group
$

The only mention of the cvsadmin group in the manpage is that the "admin" command is restricted to members of that group if the group exists. I was stuck until a kind soul explained that I should not have created the empty directory first, as even though there was no CVSROOT within it, it still counted as a "repository". A short description of the "init" command in the manpage explaining that it creates the directory itself would have been useful.