Likely they are have already created an e-voice account even if they aren't a member of your site.

They could try logging in with their chosen email address to see if that works, or requesting a "forgot password" email to set their password.

Once they have logged in they should be able to use the "join this site" link on your site to become a member.

If this fails let me know their email address (contact me at admin@e-voice.org.uk) and I'll add them as a member of your site.

Thanks

Joe