First of all, you have to be the official translator before adding one, or maximum two other translators !
You just have to click on Manage my translations in the Main Menu-> Web Translation System on the left side

then pick the one you wish and click on its title, the list of all languages for the extension appears.
Click on the "long name" of your language and the page containing the list of locale files will open You just have to click on Add another translator link that you'll find both to bottom and to top of the page

and in the page that will open you just have to select the other translator(s)

Note: you have to be logged in order to access to the Web Translation System

 

For more information, please see the other Questions & Answers