I like this add-on too, a lot spelling errors thats true but forgivable.
1. I can't delete sent invitations, it says deleted but it isn't.
2. Can you make the body text hardcoded that users can't change it? (It's very easy to delele the body text and writing "you f*cking a*shole" into the body text). That should not be possible for users.
3. Also there should be phrases for subject and bodytext, so we can use translations.