If they are refereeing outside survey it is not possible, while you can do something at end of survey, like asking questions 1.) would you like to refer survey to friends etc. 2.) form field, email address of freinds.

than you can trigger email to entered email addresses showing text like , your friend ABC refered you for this survey….

We can generate a referral URL for each respondent by appending a parameter ‘refid’ and assign response id as it’s value.

<Survey Link>?refid=${e://Field/ResponseID}

Then in the end of survey you can either follow Peeyush’s approach of asking email address or you can also show the referral URL, so that they can copy and send via whichever channel they want. Now in the report you can count the unique refid values and its occurence to find all the respondents who referred to more than 4 people and those people also completed the survey.

This is good solution @Shashi 

perfect! can you just run me through how to build out the referral link? I am not too familiar with the refid parameter function 

Also is this possible if my survey was sent through an antonymous link? 

Yes it will work for annonymous distribution. The refURL can be generated as below:

${e://Field/Q_URL}?refid=${e://Field/ResponseID}

and this won’t corrupt my responses right? Meaning, if users copy that code, I won’t get a notification they are all using the same URL. I had built it a rule that if the same URL takes the assessment it might become spam. I do not want people taking the assessment multiple times. 

Can you please elaborate on what rule you are using and are you appending anything in annonymous link URL?

I just put a rule that all participants can take the survey once, therefore, I am not sure if providing all of them the same referral link will impact that. Meaning, if Person A shares their referral code within their network, will all the people using that one code be impacted? 

No it won’t be impacted but can you please elaborate on how you have the rule - all participants can take the survey once