As long as you use a valid WordPress.org username in the Contributors: tag in readme.txt, then yes: once the readme.txt is parsed by Extend/SVN, then the specified username automagically has SVN-commit access to the Plugin.
Note: I know that it works for the Contribuors: tag, as I am such a contributor on a couple Plugins. If you're using a Collaborators: tag, I am unsure of how such a tag gets parsed.
EDIT
And I mis-spoke somewhat. @Otto reminds me that committers must also be added, via the extend\plugins\plugin-name\admin page. So, props Otto; I had completely forgotten about that part!
So, there's really no automagic. Go to wordpress.org/extend/plugins/plugin-name/admin, look for the Committers heading, add a WPORG username to the text field, and click "Add Committer".