The FAQ at Careers states:
Are the people that you invite awesome? Well, then we’ll give you new invites for each person who creates a profile with a completeness score of at least 100.
Well, I have invited several people who have created a profile with a higher score than 100 and I am not receiving any new invites.
Edit: Aparently the problem was that invitations are only issued once a week, but a week has passed and I still didn't get back any invitations.