wouldnt it be easier for your members to just send their friends your website link? I don't know hardly anyone that checks their email to join websites as emails are notorious for scams, viruses, spam etc. Your best bet is to create a list of codes like "XzC4rDf6K" to give to people. Next step is to create a special sign up field that has to be filled out with the referral code. If the code matchs the person who sent code link gets assigned points. You can then delete the code answer required for custom field so not to be used again. I hope you understand what I'm saying by creating a custom profile field requirement for invite. Also give people an option to sign up regularly just in case they lost the code etc