What is the plural noun for a group of people who registered to a website through a referral code?
Solution 1:
If that indeed is the word used, then it has no separate plural form. It is a nominalized adjective.
When an adjective operates as a noun there are usually no separate singular and plural forms.
Just as is the case with words like "deceased" "Infected" etc. when used as nouns, the base form is used in both cases.
But as suggested by those in your comments, please consider using "Referral" instead.