I've had moderate success with using the openid authorization (
https://www.dokuwiki.org/plugin:openid, or
http://h6e.net/dokuwiki/plugins/openid). Though it still requires some manual steps (including pasting the open id link
https://www.google.com/accounts/o8/id), it does let your users log in using their google account.
You have to make sure that authentication by openid is enabled for your google apps domain to use this.
It's easy to create accounts with this method and if you are already logged in to the wiki you can add the openid connection by visiting the plugin's "login with OpenID" link while logged in (i.e. yourwiki_sURI.com/doku.php?id=start&do=openid).
Hope that is helpful