The install command is not embedded into rbenv, it comes from the ruby-build plugin. You can install it using the command:
git clone https://github.com/rbenv/ruby-build.git "$(rbenv root)"/plugins/ruby-build
On Mac OS X you can install it through homebrew:
brew install ruby-build
On Debian (version >= 7) and Ubuntu (version >= 12.10) both rbenv and ruby-build can be installed using apt-get (or aptitude):
sudo apt-get update
sudo apt-get install rbenv ruby-build
On FreeBSD ruby-build is available in the Ports Collection, it can be install both as a binary package or build from the port:
# Using pkgng rbenv will be installed
pkg install ruby-build
# Building ruby-build form Ports will install rbenv only if the RBENV option is set
cd /usr/ports/devel/ruby-build
make install