When you start a new Rosetta site for your locale, you are set as an Editor with a complementary role of Translation Editor and as such, you will be able to add new users with different roles to your locale. For a better understanding of user roles on a locale check out the Roles and Capabilities page.
To add a new users, go to Users in the admin menu and select New user. Add the e-mail address or the username of the user you wish to add to your site and choose a role for them.
To enable a user to edit translations go to Users and select Translation Editors. Add the e-mail address or the username of the user you wish to add as a Translation Editor and choose the permission level for them. You can enable a Translation Editor to edit translations for all projects, a category of projects, multiple projects or a single project.