I just tried this on my demo site and it worked fine - so I don't know why it wouldn't work for you - unless maybe the site you are inviting them to is disabled or has unusual permissions set?

If you can give me any more details (email admin@e-voice.org.uk instead of sharing on here) then I will look into it further. Let me know the person you're inviting and the site you are inviting them to.

Thanks

Joe