Sponsored maintainers are people who maintain packages in the Debian archive without having any upload rights. The packages are uploaded by a sponsor.
After a while, a sponsored maintainer can become Debian Maintainer.
Anyone can create a package for Debian on his computer.
if you want to help Debian you can become a sponsored maintainer.
Becoming a sponsored maintainer
After thoses steps you will be a sponsored maintainer
step 1 : Learn to create a package
You have read the Documentation.
You know that you can join a team.
You have found an interesting package
step 2 : find a sponsor
You can use Alioth for your package.
A sponsor has helped you with it.
Once your package is on sid, you are a maintainer who has been sponsored.