As the current lead maintainer of a plugin, you are capable to add more co-maintainers yourself. Go to your plugin's main page and scroll down a bit to the "Contributors" section. There you will see a button to add more contributors.
On the next screen, you can add any moodle.org user as contributor to your plugin. If you also select the "Maintainer" checkbox, the contributor will receive maintainers rights - that is edit the plugin, release new versions etc.
Only lead maintainers can add more contributors and only the Plugins directory curators (such as myself) can change the lead maintainer of a plugin (iirc).