The Nabble registration has nothing to do with the Grails subscription. To subscribe to the Grails mailing lists, please look at the description here:
http://grails.org/Mailing+lists
You can try sending an email to the mailing list instead of using the Nabble interface. Then you can check if it gets through the list. If those pending alerts are disturbing you, you can delete the related posts (mainly because Nabble won't send them again to the mailing list).