I'll have a look.

Apparently if you invite someone more than once, you'll get that message.  I'll add it to the list so it will say something like 'you've already invited xxx'

Rob,

The always-and-only list of what-points-go-with-what is under My Points in your profile.  The direct link is here.

and is always updated with any changes to the point system. 

-Michael

Rob, I'd think that would be by design. I'm just thinking of how facebook works for instance, you get one chance to add someone as a friend or they are "already invited".  When mailing your friends, you can't force add someone who hasn't accepted as your friend.  I'd think that would be the expected way it should work.