If @rishi is going to sponsor you and has the necessary privileges, then they can just add you to the packager group. Then you can log into src.fedoraproject.org once, and then you can be added to the package as a maintainer. I suspect, however @rishi is not a sponsor.. In which case they should comment here and indicate that they want you to be added and will take care of showing you how things work. Then one of us will click the necessary button and you will be able to log into src.fedoraproject.org once to refresh the permissions and then you can be added to the package (presumably by @rishi, but technically anyone with admin privileges on the package).

Yes, I'd love to have @sumantrom co-own rpms/toolbox and container/fedora-toolbox.

It turns out that I still have my sponsor privileges intact, so I have now added him to the packager group. :)