When I use my invitation link and get on the registration site, after filling in the empty forms and sending it I get following error message:
The change you wanted was rejected.
Maybe you tried to change something you didn’t have access to.
How can I fix this issue and register myself?
I’d suggest you post this as a problem, it’s a usage question. You’ll have a better chance to get an answer/resolution: https://getsatisfaction.com/diaspora/feedback/topics/new?display=overlay&style=problem#
You may also try using another web browser.