Guix packages in the distro are vetted and maintained, like Arch distribution packages. Their number is around 31,000, so Guix is not any more a small distro.
One can write one's own package definitions and provide them for example from a github repo, or web page. That would be more akin to Ubuntu PPAs, which always have one owner.
Something like AUR which id totally open does not exist. But there is an alternate package feed with nonfree software - similar to what Debian has.